You can just get a BRU brand mattress cover for cheap, that does the job just as well as any other brand. Ours is a fabric material and stretches on like a sheet. I like it better than one that zips around because the one time I had to take it off was when my DD had a really bad stomach virus and vomited in her crib in the middle of the night. I wouldn't have wanted to deal with unzipping the cover and changing the sheets. With the stretchy one you can just reach under and pull off the mattress cover and the messy sheet in one go and contain the mess really easily. Even if your mattress is water proof (as most are) it's nice not to have to be cleaning and disinfecting it in the dark when you've been woken from a dead sleep.
I have the stretchy one that works like a sheet, and several crib sized waterproof pads. I keep several layers on DS's bed at a time, protector, sheet, protecter, sheet. Up to 4 layers when everything is clean. It makes quick cleanups very very easy. Just take off a whole layer and you're done!! I also only have to remake DS's bed once a month even though I strip off a layer each week so he's sleeping on a clean sheet.
Finally updating my signature and avatar, August 29, 2011 (better late than never!)
Jake! (born July 3, 2011 - 6 days past due)
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Re: Mattress cover
Oh, well, maybe our mattress is waterproof too....I haven't checked. I just assumed we'd need a mattress protector.