preventing shoulder tendonitis or imbalances

I had shoulder tendinitis once. I recovered, but I often feel an ache down my left forearm. Just occasionally. Today I also felt a pain in the same spot where I had my problem, the suprasinatus [sic]. This was during boxing and had not occurred to me before today.

I am wondering if this is related, or if it may be a sign my problems are developing again. I do a lot of sports. Kickboxing 3x per week, and I use free-weights, all compound movements (squat, deadlift, overhead press, bench press, chin-ups, bent over rows), 2x per week.

Is there anything I can do to prevent tendinitis developing? Is there a program I could look into? As in, a list of exercises/stretches, an order to do them in, which sets/reps and frequencies I could follow, as well as progressions if I am required to increase the weight over time.

Kind regards,
Liam