Dan's suggestions are the same thing I do. I hang my camera bag on the tripod to add stability.
Unfortunately, no matter how well we do the technical part, we're still at the mercy of the atmospheric haze and distortion, and our own failure to remember basic scientific facts like the one about warm air rising.
Yesterday it hit 80 around here. It looked like a relatively clear day. But a bad location choice on my part overlooked the fact that I was attempting to shoot a moonrise over a nearby ridge that had sun on it all afternoon. Basic science, warm air rises and distorts.
Here's an example of how not to do it. The cross like structure at the base of the moon is a 300 foot commercial microwave/TV antenna on top of the ridge about 3-4 miles from where I was shooting.