My rear brakes are wearing out very fast. I am on my third set since Christmas (2 months). I have no idea why this is. It is a rim brake.
Assuming disc brakes, some reasons for short pad life could be:
Riding conditions - If you're riding in gritty mud, pad life will be shorter than dry conditions. The mud can form a grinding paste which wears the pads quicker.
Damaged rotors - If your rotors are scored or damaged, this can accelerate pad wear. If you've worn a pad down to the backing plate this can damage the rotor and lead to shorter pad life.
Not bedding the pads in sufficiently - New pads straight out the packet need bedded in before use. This typically involves putting them through a few braking cycles to bring them up to a high temperature. Pads which have not been bedded in, will wear quicker when subjected to wet muddy conditions.
Pad material - Brake pads come in different materials which can have different pad lives depending on usage. E.g. sintered pads should last longer in muddy conditions than resin pads. However, some brake discs are not designed for sintered pads.
Not having info about your brakes, the pads you use and category you ride (AM,XC,Gravity), I will suppose your problem is your riding. You use the rear brake very often, even more than you should. You should definitely keep your brakes clean and make sure you use original brake pads, but you have to change your riding. Get used to using the front brake more often, especially if you are also riding in tarmac. Don't always lock your rear brake when using it and most important, do not use any of the brakes for consistent brakings more than a few seconds. Let them loose for short periods in between. They tend to overheat and cause great frictions between the pads and the disc/rim and they wear off faster.
Get better brake pads. In my experience, Kool Stop Salmon lasts far longer than anything else, and wears the rim very little. The Jagwire pads that I have had for more than a year seem to be doing good, too. Original pads from Tektro, ProMax etc are useless and Shimano is not very good either.
The point about using both brakes is good, too.
If you are paying a shop to replace pads every month then that can get expensive.
Clean the rims and pads regularly. Grit will wear down pads and rims.
Go with high quality pads. Mail order is going to be the cheaper than retail. Learn to replace pads yourself. Don't just slap them in - you do need to get them properly aligned.