There are a couple of solid do'a s to say when breaking one's fast. Abdullah ibn Umar was heard as saying at the time of breaking his fast:

"Allahooma innee as-alooka bi-rahmatee-kallatee wa-see-aat koola-shay-in an-tagh-fee-ra-lee"

"O Allaah, I ask You by Your mercy which envelops all things, that You forgive me."