When you're shopping for a foam mattress, be sure you are getting something that has a higher density foam. I'd recommend something above 1.8 lbs. When looking for memory foam you'll want something even more dense like 4+ lbs.

For body impression, unfortunately most mattresses will do this over time. This is even more prevalent with natural options such as cotton or wool. You may want to consider replacing your mattress more frequently and using a mattress protector that will keep body moisture (sweat, etc) from being absorbed into the core.