Can't say enough about Har-Tru on the body. Seek it out and play on it as much as possible. I notice a big difference in the feet, knees, hips, and back. Unfortunately, it doesn't help the shoulders. I'm 39 and 6 months post SLAP surgery - first major injury I've ever had. I hope it's a while before my next.
Also, I already have an eye out for my kids (8 and 6). We are a tennis family but I watch their court time, play as much on clay as possible, teach them good warm up and technique, and have them play other sports. I want them to be able to play long into their lives and I already see 12 year old juniors with stress fractures and arm problems.