A good rule of thumb is that if the rock will scratch a knife blade, then it will take a nice polish. Look for interesting rocks with pretty colors or unique patterns. For an extra gleam, try adding a tablespoon of sugar to the final polish stage.
The larger the tumble barrel, the faster the rocks will take on a nice, round shape. Make sure to put enough water in the barrel or you'll end up with such a thick slurry that the rocks will stick together.
Take notes for each tumble you do - how long each phase took, what type of rock you used, etc. Experiment with the amount of grit you use in each phase and note the results. This can save a lot of time in the future.
Add plastic pellets during the pre-polish and polish stages. This helps cushion the stones and reduce chipping. If a stone breaks during the later stages of tumbling, the sharp edges will scratch the other stones. Remove it before the final stages of polishing.
Preventing contaminants from entering the process will help ensure a good polish. It's a good idea to rinse your rocks before loading them in the barrel. Wash the barrel seal between each step.
If the rock tumbler lid pops off while it's tumbling, try cleaning the seal area. Gas build-up can also cause the lid to pop off. To prevent this, add baking soda to each stage except the final polish.
Don't reuse the rock tumbler grit because it breaks down and becomes useless after about seven days. You can, however, reuse the plastic pellets. They can be used about 10 to 12 times. Just remember to use them in only one grit. The grit gets imbedded in them, and if you use them in a different grit you'll contaminate the load.