Any suggestions on what to do for my battery?

I am having some battery issues with my 2004 Mazda 3. Last summer, I didnt
drive it for about a month, and when I went to drive it, the battery was dead.
The car would start and run fine when jumpstarted. I replaced the battery in
August, and it worked fine until last week. I went a couple weeks without
driving it, and the battery was completely dead. I jumpstarted it last night, and
let it run for a while then drove it around for a bit. Today, I tried to start it and
the battery turned on but didnt have enough juice to start the car. I don't think
the battery is the issue. Any suggestions before I take it to the mechanic?