My baby spits up EVERYTHING. What do I do?

She hardly keeps anything down (100% formula-fed) and is always hungry because it all comes back out. She is on a gentlease formula that our doctor told us to give her, and we're about to switch her back to the plain newborn formula to see if it is because of the formula (she started having this problem as soon as we changed formulas). She's six weeks if it matters.