I have had some luck using the rubber end of a hammer to tap on the dents. Gradually use a bit more pressure until you get some results. Or tap on a piece of wood on the shell. Larger piece to start and smaller as dent is pounded out.

Mostly start easy and increase power until you think it is working, or stop when you think you are hitting hardwnough but not getting results. Slow works better than single blow.

Try a rubber or wood mallet. I have used a rubber mallet designed for driving tent stakes into the ground. It works well, but like the previous post says, go a little at a time. I worked on a COB Dynasonic that looked as if someone used it as a bowling ball, and it came out great. Brass is pretty maleable, so it is not hard to work with.

Also , and this is very impotant, if you do use a wood mallet USE EAR PLUGS! as it will get very loud (just think how loud hitting a stick against the shell would be). We drummer's need to protect our hearing!