Look for ones with either PBT in the faric (Speedo Ensurance is one brand, but almost all top brands do them) OR try the new nike XLA fabric, which is supposed to be as chlorine resistant, but a nicer fabric, with more stretch and a softer feel...
I have the same pair of Speedo PBT trunks I was wearing two years ago with no obvious signs of wear and I swim three times a week in them...
djb