- Get your opponent's aggressive shot back in play.
- Give your opponent as little opportunity to be aggressive on the next shot as possible.
- Hit deep and moderately crosscourt. Unless your opponent is at the net, hitting deep and moderately crosscourt minimizes the risk in your shot and also limits his offensive potential on his next shot. If you hit crosscourt, but within the middle 2/3 of the court, you're unlikely to miss wide, and your opponent is less likely to hit a sharply angled reply, but you still benefit from hitting over the lower part of the net and having a longer (diagonal) court to hit into. Generally, the more depth on your defensive shot, the better, but if you're an average player, you'll find that aiming 3-4 feet inside your opponent's baseline will be pretty safe from missing long while still deep enough to give you reasonable time to react to your opponent's shot and to limit his ability to hit a sharp angle or a winning drop shot.
- Leave plenty of margin over the net. Most players hit their average shot too low. Hitting through a very narrow window over the net is a chosen risk when you decide to hit hard and flat, but if you're going to try such risky shots, save them for offensive situations where you have time to set up properly. If your opponent attacks at the net, your defense will sometimes employ offense: you'll "thread the needle" with a brilliant passing shot now and then. These days, though, most players prefer to attack from the baseline, and in baseline rallies, it makes sense to send defensive shots at least four feet above the net. Making sure you get the ball back is job #1 in defense, and the net is your first obstacle. Job #2 is to keep your shots deep, and the higher you hit, the deeper your shots will typically land.
- Avoid hitting down the line. In baseline rallies, hitting down the line puts you farthest from where you need to be next: at the center of the angles your opponent can hit. Your opponent can only make you run wider than your own sideline by hitting crosscourt (including inside-out), so if he is going to have a chance to hit crosscourt, you want that crosscourt shot going toward the side you're already on. You accomplish that by hitting your own shot somewhat crosscourt.
Of course, if your opponent comes to net, feel free to hit down the line as a passing shot.