When you write your dialogue, take a minute and read it out loud to make sure it sounds like something someone would actually say. Few people actually say, "I shall do something" for example. You can get away with a lot in dialogue, like crappy grammar, because people say all sorts of things, but if it doesn't sound natural then your audience will know it. The only caveat I can give you is if the character is known for stilted and archaic speech. Of course that presents the separate problem of having to remember when you write that character, to put it in his or her typical dialogue and only drop it when the character has reason to.
Bottom line - read your dialogue and make sure it is something people would actually say. Trust me on this one - I've screwed this up more times by accident than most of you could ever do on purpose.