The issue of improper tagging comes up often, both in the forums and comments, and "bad tagging" can often be seen as one of the reasons why an image did not receive moderator approval.
A good way to find out proper tags is to head over to the tag cloud:
This covers only the most popular tags, however, and tends to ignore certain obscure, yet highly relevant tags. In addition to that there are popular tags that, for whatever reason, are often "forgotten" and not added. I took some mental notes of the ones that I most often find missing (or ones that I tend to forget myself) and compiled the list below, with concise explanations copied from their respective wiki pages or written by myself for ease of use.
(Don't forget that you can access the wiki page of a tag by clicking on the question mark next to it in the Tags bar on the left.)
Edit: I suggest going through the whole thread, as a lot of tags have received quite thorough explanations here.
Tags you should use, but perhaps don't know about:
flat_chest - http://miezaru.donmai.us/post?tags=flat_chest
"girls with an (American) A-cup bra size or smaller."
between_breasts - http://miezaru.donmai.us/post?tags=between_breasts
"When an object fits or is pressed between the breasts." This is a new tag, so it may take some time for it to become popular.
paizuri - http://miezaru.donmai.us/post?tags=paizuri
The Japanese word for titty fucking, i.e. a penis between breasts.
cleavage - http://miezaru.donmai.us/post?tags=cleavage
"The depression created via partial exposure between a woman’s breasts." Use this tag only if a piece of clothing is designed to expose the breasts, not when exposure is accidental.
underboob - http://miezaru.donmai.us/post?tags=underboob
"The shirt is either so skimpy or hiked up that the bottom of the breasts are visible." This includes accidental exposure.
breast_grab - http://miezaru.donmai.us/post?tags=breast_grab
"The act of grabbing a breast with a hand." I'm not sure if this includes accidental hands-on-breasts situations or not.
breast_press - http://miezaru.donmai.us/post?tags=breast_press
"The act of pressing one’s breasts against something, whether it be a window, another person, etc."
breast_squeeze - http://miezaru.donmai.us/post?tags=breast_squeeze
"For Danbooru purposes, this refers to the act of one’s breasts being squeezed together."
symmetrical_docking - http://miezaru.donmai.us/post?tags=symmetrical_docking
"A term to describe the embrace of two female bodies so that the breasts interlock, hence “symmetrical docking”."
lingerie - http://miezaru.donmai.us/post?tags=lingerie
"Lingerie refers to undergarment worn by women. Most undergarment tags are set to imply lingerie (with the notable exception of panties, a tag so ubiquitous as to make the implication of little use)."
garter_belt - http://miezaru.donmai.us/post?tags=garter_belt
"An undergarment worn around the waist like a belt, with two straps on each side hanging down. These straps are clipped to stockings or thighhigh socks to keep them from falling down."
garters - http://miezaru.donmai.us/post?tags=garters
"Garters are elastic bands worn around the thigh to keep stockings up."
camisole - http://miezaru.donmai.us/post?tags=camisole
"A short, sleeveless top with thin straps. These days camisoles are worn as regular garments, but they can also be worn as an undergarment."
chemise - http://miezaru.donmai.us/post?tags=chemise
"A chemise is essentially a camisole that ends past the waist. Usually worn as an undergarment."
nightgown - http://miezaru.donmai.us/post?tags=nightgown
"A nightgown is a loose dress-like garment worn by women to bed."
school_uniform - http://miezaru.donmai.us/post?tags=school_uniform
All sorts of school uniforms, including the standard serafuku and all made-up and even somewhat fantastical (read: impractical) ones.
serafuku - http://miezaru.donmai.us/post?tags=serafuku
"The uniform with a sailor-style collar worn by Japanese schoolgirls in middle school and high school. The standard uniform consists of a white blouse, a red ribbon, and a pleated skirt, but there are as many variations as there are stars in the sky."
sukumizu or school_swimsuit - http://miezaru.donmai.us/post?tags=school_swimsuit
"A one piece swimsuit (usually white, gray, navy blue or black) that usually the wearer’s name printed or written in a white space on the chest area."
braid - http://miezaru.donmai.us/post?tags=braid
Several strands of hair tied together in an overlapping pattern.
twin_braids - http://miezaru.donmai.us/post?tags=twin_braids
Equally sized braids from both sides of the head.
ponytail - http://miezaru.donmai.us/post?tags=ponytail
"A ponytail is a hairstyle in which most or all of the hair on the head is pulled away from the face, gathered and secured at the back of the head with a hair tie, clip or similar device, and allowed to hang freely from that point."
twintails - http://miezaru.donmai.us/post?tags=twintails
"Generalized form of pig tails. A hair style where the hair is tied into two sections." Log suggested to use this instead of 'pigtails'.
oekaki - http://miezaru.donmai.us/post?tags=oekaki
Drawings created with an applet integrated into a messageboard. They tend to be rush jobs and have a certain element of digital crudeness to them.
sketch - http://miezaru.donmai.us/post?tags=sketch
"sketch" should be used for rough drawings; including, but not limited to, those used as a basis or study for a subsequent finished version." See forum #2088 for a more thorough explanation.
distortion - http://miezaru.donmai.us/post?tags=distortion
"... images that have been either heavily manipulated in a destructive/deconstructive manner, or were drawn from the ground up to look distorted, to achieve some sort of aesthetic effect." This is an obscure tag, but it may find some use in the future.
monochrome - http://miezaru.donmai.us/post?tags=monochrome
"Art works drawn predominately in one or two colors."
zettai_ryouiki - http://miezaru.donmai.us/post?tags=zettai_ryouiki
A short skirt + some skin + thighhighs almost always equals zettai_ryouiki. Check the wiki page for a more thorough explanation.
what - http://miezaru.donmai.us/post?tags=what
"Posts that you do not understand at all. Note that you shouldn’t tag random guro/bondage posts with this. what is for things that transcend fetishes and leave everyone stupefied." See forum #2051 for more. Don't flood this tag, danbooru is not 4chan.
Tags you should NOT use (mostly redundant if you've read the forums and comments, but still worth pointing out):
cute, sweet, sexy, erotic, hot, perfect, disgusting, violent, mature, etc. - these are all subjective and don't apply for everyone. Don't use them, ever.
ecchi, hentai - the ratings cover what these tags usually represent. I've noticed that 'hentai' is often used for images that feature sexual intercourse. Use 'sex' instead and don't forget to add the specific type of intercourse (e.g. cunnilingus).
moe - as wanchan put it, "This tag is ambiguous and will be a nightmare to untangle." My suggestion would be to only use it when the image uses the idea of moe as a gimmick of sorts (a joke on the subculture, definitions, etc.), not when the image itself could be considered moe. See post #86945 for an example.
Why is moe so confusing? Well... http://heiseidemocracy.com/2005/12/07/the-moe-image/
girls, women - Not useful, despite the plural.
A few other useful wiki pages:
http://miezaru.donmai.us/wiki/show?title=breasts - outlines and links to various (sub)tags regarding breasts.
http://miezaru.donmai.us/wiki/show?title=sex - links to various forms of intercourse and how to tag them (in need of updates).
http://miezaru.donmai.us/wiki/show?title=lingerie - types of lingerie.
How to find out whether a tag exists or not? Use the searchbox. The number of times a tag has been used and a list of related tags will be displayed under the Tags bar, even if you don't see the posts themselves.
To improve this thread further, please point out tags that
1) you most often find missing in recently uploaded posts (there are tons I'm forgetting or haven't noticed);
2) should not be used, but tend to find their way into uploads;
3) deserve more attention due to their usefulness, despite relative obscurity.
Also feel free to ask minor questions regarding specific tags here, especially if you felt that they might not have warranted a separate thread (e.g. help in identifying a certain piece of clothing). If it's a bigger issue, then make a separate thread.
The basic purpose of this thread is to help newer users find out about tags like zettai_ryouiki and symmetrical_docking, raise awareness in regards to female clothing, and to make sure that they don't mistake breast_squeeze for breast_press. Correct tagging reduces the workload for everyone and makes the database more complete.
If this thread starts serving some purpose, then we can compile a list of the most commonly forgotten tags and perhaps even create a specific wiki page for it. It might also serve as a proving ground for a wiki-based FAQ for danbooru, seeing as the current help pages are all about danbooru usage in general and rarely (if ever?) about the particularities of this specific danbooru.
If not, then... well, at least I tried. orz
Updated by Elfenus