In general, I find that aiming slightly "above" something on the iPhone/iPod Touch works pretty effectively. Even with my pretty big fingers I generally don't have any problems.

Also, I've noticed that people with long nails (who tend to flatten their fingers when tapping) have a hard time tapping accurately. I don't really have any suggestions there, unfortunately. :-/

The other trick is to avoid the multiple-tap process of getting to the "Complete" button. Instead you can do a small swipe on the right side of an item in a task list, which brings up a little "Complete" button to tap.

guanolad, I think jstaso was talking about the Incomplete/Complete buttons when trying to swipe to complete a task, as you don't see the Today/Tomorrow buttons when editing a task's details.