Based on some frequently asked tag questions lately, Kurt asked me to post a quick DOs & DON'Ts guide on how to get the most out of the tags system. Great idea, Kurt! This guide applies to both art submitters and people searching for art. This guide only applies to tags; not title, description, or other search fields. :)

The goal of tags is finding what you're looking for. Think of as many single-word ways of describing what you're submitting/seeking. Although there are situations where multi-word tags are needed, the overwhelming majority of tags are single-word descriptors. Try to enter several single-word tags instead of fewer multi-word tags. "dark, fantasy, rpg, rigged, static, mesh, lowpoly" is more effective than "rigged lowpoly mesh assets, static 3d meshes, dark fantasy rpg art". 

DONT:

• #hashtag - OGA tags are not linked to twitter. Please don't use hashtags when submitting or searching by tag. They just make the artwork harder to find since "#sword" will not show up in a search for "sword". They also break alphabetical sorting since #ZXY comes before ABC.
• Omit commas between tags - If you don't put a comma between your tags, people searching will find nothing, and people submitting will never be found. "do not forget commas between tags" is all one tag that has to be matched exaclty >:( "do not, forget, commas, between, tags" is 5 different tags that can be searched or matched individually. :)
• ennemy, midevil, enviroment, sord - Be sure the tags are spelled correctly for best results. (I always misspell "medieval")
• Over 20 different styles!, Mega-Awesome Studio Pack v1.14881b, Haha! :), boring artwork I made, I need artword that doesn't suck - Who would ever submit or search for these phrases?

• free, open source, video game, art, asset - Everything on OGA is a free video game art asset. These words won't help narrow the search.
• cc0, cc-by, gpl, Creative Commons, Public Domain - Tagging the license is unneccessary. There is a separate field dedicated to searching by specific licenses.
• texture, 3d art, 2d asset, music, sound effect - Tagging the art category is unneccessary. There is a separate field dedicated to searching by specific art category. Don't put "string music, dark 2d art". Just put "string, dark"
• swords, rocks, icons, cacti - Plural tags will not show up in a search for the singular version. Searching for "sword" will not show any artwork tagged "swords" and vice versa. When in doubt, just use the singular tag (sword, rock, icon, cactus) Even if the asset contains more than one icon, many swords, a big pile of rocks, and a whole forest of cacti... the singular form of any tag is far more commonly used. You could include both forms, I guess, but it will keep things a lot simpler if you try to stick to a singular convention. The exception to this is when the singular form has an entirely different meaning than the plural. Pants ≠ multiple pant, glasses ≠ multiple glass.

• Case SEnsiTive - UPPER-CASE, lower-case, makes no difference. When in doubt, though, put everything in lower-case.
• artist/submitter name in the tags - Adding your name or your project name as a tag is unlikely to help anyone find the asset and has zero SEO impact. There is a separate field dedicated to searching by the artist's/submitter's name.

DO:

• orchestral, chiptune, trance, portrait, synth, pixelart, chibi, anime, painterly - tags that indicate the style of art are useful.
• 32x32, 8-bit, 16-bit, dawnbringer, db32, 128x64, 44.1 kHz, 110bpm, specular, bumpmap - give common technical specifications of the asset, if any apply. Being able to search by dimension, palette, sample rate, et cetera is helpful.
• 3/4 perspective, overhead, ortho, 1st person, isometric, oblique, sidescroller - include the art's perspective, if applicable. Different games have different viewpoints. Being able to find the right kind is invaluable.
• sword, dog, face, warrior, robot, grass, rock, sad, spooky, upbeat, relaxing, western, fantasy, scifi - a few one-word descriptions of what is in the asset will go a long way.
• gui, ui, animation, animated, sci-fi, scifi - Try several versions of tags with very similar meanings. Sometimes people try one and don't think of the other. This will allow the search to work either way. No need to go overboard, though. "gui, ui, graphical user interface, user interface, graphical ui, u interface, user i, gu interface" is just getting ridiculous. Just a couple of the most commonly used tags is enough. Not sure which are most used? search for the ones you're thinking of and see which one returns more results.
• .png, .blend, .obj, .wav, unity3d, .svg, .flac, .xcf - tagging the file format is often useful for people working with specific software suites or working with specific programming constraints. For the sake of conformity and ease-of-searching, please always include the dot before file extensions. Otherwise people searching for Photoshop ".psd" files will never find the asset tagged "psd".
• Edit your submission - Don't be afraid to click that "Edit" tab at the top of your submission. If you misspelled something or forgot the commas (I always do), no need to delete the submission, just click edit, fix whatever doesn't look right, and click save. easy peasy.

TL;DR: A good rule of thumb I use to test if a tag of mine is good or not: If I search for any single tag I've used, but my submissions- no one elses- are the only results... It's likely no one will ever find my submission using that tag. I should probably change or remove the tag from my submission. If I'm the only person that thought of this tag, I'm the only person that will ever find my art.