What causes the heater core to go out in a 1998 F150?

ok i have replaced the heater core twice now in my 1998 Ford f150...and once again..it has gone out..what could be causing the heater core to keep going out? the problem i have is i replace it and then it leaks antifreeze from the heater core box (where the heater core sits)..is there something i can do to prevent this? it isnt much fun on a cold morning turning on the defrost only to have antifreeze blow out instead of heat lol ...so any any any advice on what do to would be great..cause this time when i change it out..i would like to prevent the leakage and having to spend time to change it again..thanks so much!!!!