Again, look for firmness when deciding on a mattress. Firm mattresses prevent your baby from becoming trapped between the mattress and the crib sides. Consumer Reports suggests that parents spend a little more money on a sturdy crib mattress versus opting for a flimsy, cheap model. That doesn't mean buy the most expensive mattress. In fact CR advises parents that labels can tell you only so much. You have to get the mattress out of the package and test it to determine its firmness and that it fits properly in your baby's crib.
While you're testing your mattress's firmness, also check its height. Most mattresses are 6 inches tall, but some models are shorter. Ill-fitting crib sheets can come loose and potentially strangle or suffocate a baby. The Consumer Product Safety Commission (CPSC) admonishes parents to pick a tight-fitting crib sheet that overlaps the mattress and can't be pulled aside. Again, testing is key. Try out the crib sheet on the mattress in your baby's crib. Make sure that the sheet can't be pulled off.
You may also want to consider a fire-resistant exterior fabric. Serta offers what it calls FireBlocker protection so that if a fire does reach the crib mattress, it won't spread, allowing parents time to get a baby out of the crib. Serta uses this fabric in all of their crib mattress models, both foam and innerspring.