sounds like you have unproductive keywords. check your bounce rates with consideration to time-on-site for those keywords to decide which ones to keep. then, do more keyword research to find more productive keywords. add those, rinse, repeat!
Funny how "bounce rates" can be totally wrong... I mean if I find a website and land on it and it offers me something I want for an e-mail I'll enter it in less than 30 seconds, most of the time...? That's usually what the bounce rate is? Isn't it... May be wrong here.. - Chris
actually, that would fall under the conversion category, not bounce rate. bounce rate is for those who hit the back button, leave by entering a diff url, or close their browser
You could also just have a couple winners in there but the losers are sucking your budget dry. Hopefully you have some kind of keyword tracking in place or are using googles tracking to see your cost per conversion. Sometimes eliminated the budget draining and unproductive keywords will turn a campaign around.