Categories aren't tags[]
Categories on wikis should not be mistaken for the generic tag system used on other websites like blog sites. Using real examples, HUNK should not be categorised under "Raccoon City". That category is intended for buildings and districts within the city, not for anyone or anything that has ever been involved with that city. The category would be too large to use reasonably.
Double-checking[]
Remember to double-check that the category you are inserting is the right one. Some categories are spelt in a similar manner to their proper duplicates. There can be many reasons for this; it could be that the category name was changed, but articles were still left under it: "Category:Male Characters" and "Category:Male characters" both existed for a time before the former finally ceased to exist from lack of usage.
When adding a category, remember to check the grammar. As this wiki uses the Wikipedia method of titling - "if it's not a proper noun, don't make it look like one", words like "characters"; "items" and "weapons" should not be capitalised unless, of course, that is simply the category's name (eg. "Category:Characters"; "Category:Items" and "Category:Weapons", respectively).
Don't over-categorise[]
Some category pages are not intended to be used en masse, as a complete category would make it too large to properly search through, turning it into a paradox. Jill Valentine should not be sorted under "Category:Resident Evil 5", but as "Category:Resident Evil 5 characters" - you don't want to be looking for characters and only find weapons and walkthrough guides.