Keeping your characters consistent can be a difficult thing. A lot of people have trouble with continuity, and a lot of people I know personally have a problem where their characters all read like they're the same person and it makes them difficult to tell apart. So I'm going to share some of my dirty little tricks that help keep your characters apart and make them read like separate people even if it's cheap.
Give them a favorite snack. - Food is a really cheap and easy bookmark for "this is that character". I have a character I RP as in my play by post gaming group that, whenever she is on screen and there is food consumption going on, is drinking Dr. Pepper. This fact is so consistent about her that when she was kidnapped and replaced by a doppelganger the first real hint that tipped people off that it wasn't her was that it got a different brand of soda to drink. Another character of mine is always eating spicy foods and munches ghost peppers like they're potato chips. It seems like a small thing but eventually people will notice the pattern and it will make your character seem more real to them.
Pick out some unusual words for their vocabulary. - A friend of mine once had a character who was always talking about how "awesome" everything is. In that exact word. I had a friend IRL who referred to everything as "great". This goes for the negatives too. Find an unusual word that you don't ordinarily use, staple it to that character in particular, and it will create the illusion of them having their own speech patterns.
Remember what they are afraid of. - A fear of the dark or enclosed spaces, when kept consistent, can be an amazingly powerful tool for keeping your character consistent. Whether it's a debilitating phobia or just something that makes them nervous, it will stand out and people will remember it. Even better, it counts as a flaw of sorts and makes them more relate-able because everyone has something that scares them to one degree or another. It's a good way to make them human.
Remember what they geek out over. - Everyone has something they can talk about all day. If your character has something like that not only does it make them read more consistently but it gives other players something to approach them about and it makes it easy to get into more meaningful conversations with other players's characters. In a book, the second this thing comes up your reader will instantly know your character is about to spazz and make them feel as though they know them better.
Give them a berserk button. - Something that can always piss this character off. Does she hate being called short? He got a problem with women in positions of authority? She just really not like seeing other people eat meat? It doesn't have to be a justifiable thing, you can use this to give them a flaw after all. Just be careful if you pick a bigoted thing to be this because inevitably it is going to make them look like an asshole in a way that is really hard to recover from.
I know this all can seem really tropey, but the tropes are such for a reason. There's no shame in going quick and dirty with this stuff, no shame in using the short hand in order to give yourself a leg up in actually building your plot line and story details instead.
What I learned today: The ASL signs for "job" and "rough sex" are one flipped hand away from each other. Oops.