The issue your suffering from is known as "memory leak". Functions of your device such as apps use memory when they are in use and are suppose to release them when their closed. Depending on the os your using some are more stable then others at releasing memory when their no longer in use.
When I was using os6 I would frequently dwindle down into the 30's, for this reason I downgraded back to os5. Under os5 I'm stable at 115.
Only thing I can really say is close whatever apps your using when your down with them {return key instead of end call button}, battery pulls often, try another os or even downgrade to os5.