You should not be having to jump/change your battery so often. However, each time the battery dies, it loses some of it life. There's a few possibilities. You've already checked for corrosion. Maybe there's something wrong with your charging system like others have said. You really need a multimeter to properly diagnose electrical problems. Your battery should be between 12-12.6VDC when the car is off. At idle, it should be between 13.5 to 14.7VDC.
You can remove your alternator and take it to Advance Auto to have a tested. It needs to be removed from the car. They have a machine that they can put it on. This is a free service.
If all that checks out you probably have some type of parasitic loss when the car is off. What mods do you have? Sometimes when people install head units, remote starts, security, or other electrical mods, they end up wiring it incorrectly and causes the battery to drain even when the car is off.