Trousers wear at crotch?

My trousers wear out and the crotch very quickly - sometimes in matter of months, even if I wear the trousers just once a week. This is not good given the rest of the trouser is fine. Any suggestions on how to deal with this and prolong the life of trousers? Ive heard there is iron-on material available to reinforce trousers, where can I obtain this?