I always use two ziploc bags (put things into one, seal it, then put the entire bag in another ziploc and seal that one). Have never had a problem.
As another poster said, pack them in the middle of "squishy" things, like clothing, not by harder items.
Make sure you don't pack bottles that are completely full to the top; leave a little empty space at the top of the bottle, to compensate for air pressure changes.