Amanda G. Stevens
I’ll try to be a little more original than “keep writing” and “read all the time” (though both these are necessary!). It's vital that your characters are unique and realistic. A few ways to achieve this ... Prevent stereotypes by creating contradictions in their souls. Dig deep into their pasts until you know not only what they do, but why. Pepper them with unique individualities: what music and food does she enjoy, where does he feel most relaxed, what does she do with her body when she’s nervous, what would he rescue from his house if it was on fire? For me, knowing these things is an intuitive process (Stephen King says it's like unearthing dinosaur bones). I never really feel like I choose who the character is inside; it’s more like I discover what’s already there.
Another important thing is to listen to people around you. Dialogue voice is perhaps the single indispensable key to writing individual characters. Train yourself not to hear voices but to see them. What does actual speech look like on the page? After you know that, you can mold it into fictional speech.
You’ll also need to be able to edit yourself without pity, and the only resource you need to learn this skill is Self-Editing for Fiction Writers by Browne and King. My advice is to read this book and do what it says. Without pity.