Duke,
That is one of the main reasons I had to give up window cleaning each day. I had tennis elbow in both arms. About the only thing that really works is rest.
You can treat the symptoms, wear braces on your forearems, get cortizone shots, take anti-inflamitory meds, do special exercizes, but it is painful to have to deal with.
That is the main reason I began to train people and do more in the area of management.
Sometimes if I do a large job, I will feel it in my tendons the next day. Especially a lot of scraping will do it.
Hang in there; maybe rest your arms when possible or hire a helper?
Take care,
Texas Girl