The first step was something I discovered after reading a rant on fantasy novels - characters with names in invented languages blurred together for the rant-er, so when I wrote a fanfiction novel for NaNoWriMo, I kept a list, like this advice, uh, advises.
It really did help both on rereading and on writing (so that I didn't accidentally say one and mean another), even though the only letters I managed to have only one use for were O, Q, and U and none of those were a main character! The four main characters at least didn't share a letter (there was an A, a D, an R, and a T). I excuse this behavior by saying there were almost eighty named characters, and though most of them said one line and vanished, or were only mentioned in passing, they still needed names. :[

[Edit: and I forgot my primary reason for this comment, which is to say "thank you for reminding me of the character-contest!"]