@raphjd:
Get a top of the range mattress protector, that seals the entire mattress inside. Also make sure it's waterproof so any spills don't soak into the mattress.
The average mattress should last 8 - 10 years, but check the label as some are only good for 3 - 5 years, while some are good for up to 20 years. How long your mattress lasts also depends on whether you follow the instructions on its care or not. Also, if your weight is classed as "non standard" (meaning: you're overweight) then your mattress will need to be changed sooner.
The others are correct, only steam your mattress to clean it. However, you can also do a light Fabrise (sp?) to freshen the smell.
BTW: I know all of this because we just bought a new bed. :cheers:
I am definitely investing in one for the future lol. I've never heard of a mattress protector honestly