Both of my children still sleep in cribs. When the second crib was given to us (passed down from a friend), our friends had put a piece of cardboard in the bottom to protect the mattress. I wish I had thought of that myself!
Here is the mattress that we bought (brand new) 2 years ago for our oldest child. After 2+ years of use, the metal wires has put indentions in the mattress (thankfully, it hadn't punctured the vinyl) and even some rust streaks.
Step 1: Use a paste of baking soda and water to clean the mattress. Most of it came off for me. Below is the mattress after I washed and scrubbed it.
Step 2: Cut a large piece of cardboard to fit in the bottom of the crib. Here you can see the cardboard. The bed on the left already had cardboard; the bed on the right was the one which needed the mattress cleaned and cardboard put in.
Step 3: Get children out of mischief, because by now, they have probably gotten tired of jumping on the mattress and playing with card board scraps. ;)
Step 4: Put clean sheets on the mattress and lay it in the crib, on the cardboard. Here are the finished beds! Yay! :) Now I can expect my crib mattress(es) to last a lot longer. :D
For more helpful tips, visit Shannon's blog! :)