iPhone battery runs down frequently?
Based on the "iphone" tag on this question, I'm going to assume you're talking about an iPhone battery.  The lithium-ion batteries used in portable devices have a limited "lifespan" (number of charge cycles) -- their maximum charge capacity diminishes slightly every time they are charged.

Apple has published tips on battery maintenance as well as general tips to get more life out of iPhone batteries.

Unfortunately, iPhone batteries cannot be easily replaced by consumers, but Apple will replace them for $86 with a one-week turnaround.  There are also third parties that offer to do this cheaper, but it may affect your warranty if they are not authorized technicians.

Finally, if you are really into DIY, you can buy the parts online and replace it yourself (about $30 including tools and shipping), though some have suggested that this is a difficult procedure.