TL;DR...Use the links I've provided in the text if you're interested in what I've researched. The best solution I've found -- hands down -- is using a rosin bag. It is cheap at $8.21 on Amazon.com, does a better job than all other methods I've tried combined, and lasts for several games! I simply powder my paddle hand lightly prior to my first game and it's good for six to eight games in my experience.

Longer Version

I'm a competitive 3.5/4.0 player and have been playing about two and a half years with a racquetball, ping pong, and some tennis background.

I've used several different techniques/methods to keep my paddle hand dry: Towels, new grips, overgrips, racquetball glove, etc.

I've been in the habit of keeping a towel with me. I really like the towels at $19.95 for a pack of two. I've used the towel between points to dry my hand and grip. I've also gotten into the habit of drying my grip on the front of my shirts for a quick dry off. Between my shirt and towel, I've had decent results, but once you get several games into open play or a tournament they start to lose effectiveness.

Since I've started using the rosin bag before my first game I've noticed that I haven't needed my towel or shirt nearly as much -- if at all -- and I've found that holds true through several games in even pretty hot, outside conditions.

I hope others find this helpful!