Just buy from a solid reputable source. We bought a new bed from Warren Evans about 18 months ago and specced a medium-firm mattress. After about three months it started to get softer, to the point I was getting lower back pain. Then Warren Evans went tits up and out warranty went with them :(
Oh great cloud mind, please recommend a mattress that you've owned for years and still like to sleep on.
Requirements: Firm, won't soften up so much that you feel like you're sleeping on Mark Francois' brain.
Any love for IKEA? Our last one cost a fortune, price does not predict durability it seems. Our current one has gone from firm to medium in 2.5 years.