help with male guppies

I have been having some problems with keeping my male guppies alive. When the
tank was first set up I started with 1 male and 1 female guppy. They lived
for about 3 months then both died a few days appart. So I went to PetCo and
got 2 females and 2 males. The males then both died a few days later, the guy
at PetCo said it was because the males fought for the females and killed each
other. So then I only bought 1 male to put with the 2 females and he just
died and I only had him for less than a week. It is driving me crazy! If
anyone has any ideas to why this is happening or what to do, please help!