Following through.
As a parent, you might say "We'll do that later" or "You can have that after you wash your hands." You might promise that home is the only place you're going after Walmart...
Deciding on an extra trip just because you're in the area or not giving your child something s/he wants when you said it would happen is one way to break your child's trust in you. To prevent that, just be sure to follow through and only promise what you can!