There's a *lot* of elbow tips already in this forum - and a number of good round-ups if you use the 'search' function - so here's a little perhaps contrarians advice.
Besides all the ez stuff like avoiding 'poly' strings and light racquets, technique is a critical part of the problem. Folks spend years hitting the ball a certain way and because they are on one side of the genetic age equation they can get away with it. Until they get on the other side; compensation patterns break down and suddenly the weak link is staring at you in the face... er arm.
You'll find many good lifting tips. And almost all of them can *give* you tennis elbow/golfer's elbow. Lifting like a madman can exacerbate things significantly.
The tincture of time is nature's most powerful weapon (that, and for the tennis player, learning proper technique...!).