style - How do you avoid the problem of all the characters in your story sounding the same?

There's a common flaw that I can easily detect in amateur writing, (and of course my own) which essentially boils down to all the characters "sounding the same", dialogue-wise anyway. Trying to overcome this problem, however, I find myself going to opposite end of the spectrum, overcompensating, adding so much accent/slang it gets annoying. ("i fancy me some cider, what ya'll say we pounce on over to the ol' waterin' hole", etc). How do you give each character his/her own unique voice without making it sound forced?