My uncle told me to use warm water once.  I didn't want to believe him, cuz he's kind of a douche in most aspects, but he wasn't entirely wrong here.   

I'm not convinced it really makes a huge difference on good moist herb, but when I'm smoking really dry grass, I definitely notice the difference.    Someone else mentioned moisture, it's probably just a wetter hit.    

It's worth a try.    I also like to squeeze a bit of lemon juice in the water.  It doesn't add much of a flavor, and keeps the tar from sticking to the bong.