Write for your characters. Don't become them. If you 'become' them, they will start to merge. Do not transfer your or your other characters like and dislikes on a new character. If you see someone you know with one character, do not 'try to think of a reason' to meet them with a new character. Roleplaying is no different than writing a story. Create a personalty and stick to it. A good way to do this is to base your character on another character, be it from a book, real life person, or TV character. Try to imagine what they would do.
I have had over 20 characters, each with a distinct personality. Some of them are a bit similar, but that is unavoidable and quite all right. It would also be unrealistic to try to make your characters have nothing in common at all.
If you find that your characters are becoming similar, here are a few little exercises you can do.
1: Create a small situation and write two short stories, one for each of your characters. If your characters react the about the same as you write them, that is ok. However, if they both -think- the same way as they react, you need to step back and reconsider if their personalties are the source of the reaction, or if it is your own personality.
2: Write another small situation where both your characters meet each other. Give detailed text on what they are both saying and thinking, as well as physical reactions (blinking in surprise, shrugging, etc). Perhaps they have an argument, or are trying to make a decision about something. Don't put in any names about who is thinking or saying what, and do not give descriptions. Do several things with the text when you are done.
A: See if you can easily tell who is who. You may share the text with a friend if you want.
B: Put names in for your characters, but switch them so that one character is doing all the other's actions and thoughts. As you read it, try to picture if it makes sense, or if you have the reaction of "<name> would not think that. This feels all wrong." If you do not get that reaction, then you need to reconsider your characters.
3: While RPing ingame, consider your other character you are not playing at the time. Think to yourself "How would he/she react to this? What would he/she think?" Make sure the reactions are always slightly different in your mind.