They say that if you don't replace your mattress, after about 10 years, it's weight would nearly double. Yes, it's a bit disgusting to think what all those added weight comprise of. That's why you should replace your mattress every 5 to 7 years.

To prolong the life of their mattresses, some people use mattress pads, which are usually tightly woven sheets that provide a barrier between you (and the debris you shed off) and the mattress. It prevent the proliferation of mildew and dust mites. Now these protective sheets are not cheap, either, so you'd want to take care of them properly. Here's an article with tips on how to take care of your beddings. For mattress pads in particular, it says:

If you have latex or memory foam pillows or mattress topper, you need to hand wash these items every 6 months or so. Once again, use liquid detergent instead of soap, as soap is much more difficult to remove. Squeeze the foam pillow or mattress topper until they are full of the sudsy water. Then rinse until clear, being careful not to twist or wring the foam. If you use some vinegar during the rinse process, the detergent will dissolve much faster.

I'm not sure how long a mattress pad can lengthen the life of your mattress, though. I'm still for changing every 5-7 years. After all, dust mites or none, the springiness of a mattress does suffer somewhat after several years.