Use a spare sheet of cheep paneling or drywall and practice your technique before you start your bedroom wall.
Your base coat for the sponging should have some sheen to it. At least an eggshell if not a semi-gloss.
When you do the sponge coat, thin the paint with some water and also a product called "Floetrol". You can get floetrol at any paint store. This stuff will make the sponge coat dry slower to give you more time to work with it.
Beyond that, its a matter of practice and patience.