There are basic guidelines that you can follow.
1 ) Most balls are easier to send back cross-court. So, send most of your balls, against most opponents, cross-court.
2 ) When you are well balanced and properly set, it's easier to change directions.
3 ) The more powerful a stroke, the easier it is to change direction. If you are a forehand-leaning player, your forehand being more powerful, it will be easier to change direction with your forehand.
4 ) When the path of the incoming ball does not cross you in front of your body -- that is, when the ball moves towards you and not away from you --, it's easier to change directions.
5 ) When the ball falls slightly short or is not peculiarly commanding, it's easier to change directions.
6 ) When the ball is to be struck closer to your ideal strike zone instead of being peculiarly high or peculiarly low, it's easier to change directions.
And always remember that hitting down the line means you will have to cover more ground before reaching an ideal recovery position. That is to say, expecting a cross-court reply, you will have to move to the other side of the court after you have struck the ball down the line. So, if you do not have acceptable running ground strokes, you have be even more picky about which ball whose direction you will change.
Now, for different shot options, there are guidelines too.
7 ) Tennis has four dimensions -- that is, three directions and time. It means that there are times where hitting shorter actually means leaving the ball even further away from your opponent; and it means that you should not think of it as simply running your opponent from side to side -- there is also depth, height and time that you can play with.
8 ) Don't drop shot if you're moving backward. Drop shot when you are slightly inside the court and when your opponent is far off the court.
9 ) The more spin you put on the ball and the more height you give to it, the slower it must travel to be a safe shot. So, make sure that your average ball is somewhere in between a nice arcing ball and a flat winner.
10 ) You can change the tempo of a rally by using a nice loopy shot or slices. But don't overdo it; the point is to make your other shots harder to anticipate, not to dink all the time.
11 ) Never pull the trigger when you are not standing inside the court unless the court is already empty. The further away you are from the net, the harder it is to produce an angle or a solid shot. If you want to nail the ball, wait for a shorter one.
12 ) The last thing you do is planning to take time away from your opponent. That is to say, it's always your last tactical option -- the one you revert to when your opponent is good enough to command it. You start by doing easy stuff to cut down on errors; then, you can try to exploit their weaknesses; and, if neither works, you try to make them run. It's only when nothing else is enough that you start going for half-volleys, hitting on the rise, charging the net, etc. on a regular basis.
13 ) Because point 12 was important, I will echo it again. You never want to do too much to earn a point. Excellence lies in using the right measure, Aristotle said. If you all you need to earn points is to wait for your opponent to make a mistake, don't make them run from side to side on top of it. You'll be making mistakes that are completely uncalled for.