Screenwriting : Character dialogue by Brian McAndrew

Try writing out a list of your characters and write down each character's core characteristics (I would list 3 or 4 for each). These should reflect the character's personality. Is your character wise or manipulative or generous or self serving or whatever... (These characteristics can sometimes be contradictory within a character.) Then, as you get ready to rewrite a scene, refer to your list for the characters in that scene and rewrite your dialogue with those specific characteristics in mind. This technique helped me a lot. Maybe it will do the same for you. Good luck!

Perhaps it's not as bad as you think it is. Gather a few friends and have them read the script out loud. If you can, use actors.

Know the characters. If the characters are individuals they will speak and behave as individuals. If all of the characters sound the same, that's potentially a much bigger problem than just dialogue.

What William said

Think about their' backgrounds and how those would effect their word choice, dialect and inflection. If you want a second reader, please send me a message. I'd be happy to take a look at your script and provide notes if you'll reciprocate.

Oh. No...

Let your characters breathe life and look from the outside in. Maybe that sounds weird or maybe unrelatable but I'm getting to the point that if what I write doesn't breathe a life of its own including characters I just scrap it. You have to see them breathe life, well I do anyway and I don't think there is a technical fix, the technique however is letting and seeing them breathe life.

All of the above is good advice on making sure the characters themselves are distinct in personality, motivation, etc. I've found my dialog is better when I cast the movie in my head. Pick your voices and actors, even if you never mention to anyone who they were, just to borrow their inflections, tones, pacing, etc.. Granted, if you get too specific or rare, like a comedian with a very specific style, cartoon character, etc. it could skew things poorly, but they will sound different when you're done.

Perhaps it's because my first love is acting, but I get into character to write each person's dialog. I've mapped out who they are and what they want, and how their past influences the way they behave, then I just have fun with it. And, as Shane said, sometimes casting the roles in your head helps.