Steven Hoober has some great guidelines for touchscreen design in Fingers, thumbs, and people:

Most of all, within what you can control: Always design for hands, fingers, and thumbs.

And remember: You don’t design for iPhone or Android, for cars or kiosks, for Web or apps, but for people. Have empathy for users and respect their choices, their ways of working. Account for the limits of their lives, their environments and their abilities.

Even when your implementation is constrained by technology, avoid designing for pixels or code; always consider what effects your work will have in the real world, when people look at, hold, and touch the screen.