Although I'm admittedly biased, Get Satisfaction's Overheard feature is pretty cool. I use it to reach out to people who are having problems using our site.
One way to monitor Overheard, apart from going to the Overheard tab for a company, is to set up an RSS feed. You can do that by clicking on "Subscribe to Overheard".
But, here's the real tip: When you want to reply to a tweet, your RSS reader will show you the "Reply to" link. Like so:
This way, you don't have to dig through Overheard topics. You can just get in there and start helping -- quickly.