1. You only need to link to a page once per page. E.g. you don't need to link to Leonard every time his name pops up on Penny's page. Just once is fine.
2. Thankfully, I haven't been running into this one too often, though I used to see it all over the place. When you're linking to a page, don't do it like this: Leonard lives across the hall from Penny, his girlfriend. Do it like this: Leonard lives across the hall from Penny, his girlfriend. Embed the link only in the text that it relates to. And no, just because a word follows the word you're linking to doesn't mean it should have a link stuck on it too. (I'm probably going to remove the link regardless in accordance with #1, so this one doesn't really matter.)
Extra spaces in between sections and words do not belong on pages. No explanation needed.
4. When editing or creating an episode page, avoid mentioning the same event more than once. If it's a minor event that doesn't really pertain to the plot, stick it in Trivia.
5. Avoid very long paragraphs. It's easier on everyone's eyes if you split them into two or more paragraphs each.
6. You don't need to mention that every episode on this wiki is a Big Bang Theory episode. Since this is a BBT wiki, it's pretty much implied that if a TV episode is mentioned, it's a BBT episode.
7. Not really a mistake, but I've been seeing some categories that should either be deleted or merged with others. I don't know if regular users can do that, so if a moderator or administrator could check on that, it'd be great.
These only take a few extra seconds each to implement, and they'll make everyone's job easier. Please consider them for the sake of accuracy and ease of reading.