Nina,

I know some stick framers who have gotten relief by wearing the forearm braces that are sold for tennis players. You might also want to evaluate how you are holding the mallet. Some people have a tendency to place their thumb along the handle (to direct the blow) rather than around it and this can cause problems.

There is a ton of information on the Internet about tennis elbow; accessible from any search engine. Here is just one: http://www.tennis-elbow.net/tenniselbow.htm