One thing I've learned from rpgs, is that using an accent can go a long way towards feeling like your in character. If you can't pull off an accent, then just changing your speach pattern can have a similar effect. If you're a medieval peasant girl, use old English. If you're an android, speak in a monotone voice, and don't use contractions. If you're a pirate, talk like a pirate. You get the idea.