How much are you willing to spend? I got a good pair of Sony over-ear phones (these ones: http://www.amazon.co.uk/Sony-MDR-V150-Headphones-Reversible-Monitoring/dp/B00005QBU9/ref=sr_1_1?s=electronics&ie=UTF8&qid=1296653101&sr=1-1 ) for £6 (yay Asda), and they've lasted a good long while without any deterioration of sound or damage to the actual headphones.
If you're looking for something more expensive (as in over the thirty mark) I'd stick with well known brands, or those known to cater for professionals (DJs and such).