The wall, as I realised, offers a lot of different practice options. When playing against a wall, you can work on parts of the game, that are otherwise very difficult to master; feeling, reflexes, movement, awkward situations, half volleys, coordination, precision.
I actually imagined Aga Radwanska or Bernard Tomic having done much wall hitting when they were young, cause I looked like on of them when I was playing the wall. Well, not as good, obviously, but I really played a lot of awkward-looking shots and also hit very flat.
I was forced into directing the ball well, not only because shapes were drawn on the wall into which I was supposed to hit, but because I realised that if I hit too high, too low, too left or too right into the wall, the wall is going to retrieve my ball even higher, lower, more left or more right. So in order to keep a long rally, I had to be careful where I direct my ball. I also needed to hit with the right speed and spin when wanting to "keep the ball in play."
2.) HALF VOLLEYS
Once I got into the rhytm, I totally loosened up and started thinking of things I could improve when hitting against the wall. I figured half volleys would be a great shot to improve, because many times I am forced to play them during a match and not that many times I am successful with them. Half volley is in its core a very awkward shot, not much so when you get a nice on under your feet on either forehand or backhand site, but more so when you don't have enough time to move away from the ball and when you play against a hard hitter where you're almost forced to play half-volleys from the baseline. And on the tennis court, I played so little half volleys that playing 200 half-volleys in a row was a great exercise for me. My reflexes worked well and I kept my concentration point on that circle that was drawn on the wall, trying to direct my half-volleys there; that actually kind of calmed me down.
Also, what I tried to do is playing spin half-volleys combined with flat half-volleys. I would play 5 minutes of spin half-volleys and then 5 minutes of flat half volleys. Spin half-volleys should really help you improve your return and your anticipation and concentration. The only problem is that my tennis wall was near a river and after 50 half-volleys my head'd gone berzerk and the ball'd flown into that river. And don't imagine I went swimming after it.
Actually I figured working on your return is the same as working on your half-volleys only you step about 20 steps backwards. When you feel you've loosened up your arm (sometimes you must do a lot of loosening because the wall makes you play so many balls in such a short amount of time that you can have problems with adapting to the speed, my arm was very flat at first), you step backwards and start to gradually increase the speed of your shots. In the end, your shots should be played at 90%+ of your power and consequently your backswing should shorten. Even when playing a bad server, it's good to have that immediate reaction, doing something with the ball straight away. I think this exercise is also very important for a mindset of a player, because it keeps you active and attacking. It forces you into moving your legs and setting up quick for your shots. But on the other hand, this gruesome hitting into the wall also makes you play shots from a bad position. Sometimes you are late on a ball and the wall retrieves you a ball straight at you (that way, you have to play your shot from a low-balance position) or far to the right, making you play a slice or a tricky one-hand shot (maybe you can even try playing a other-handed forehand like Sharapova). Anyway, you have the right to remain creative.
Oh, what else: Try combining the half-volley and return exercises. First blast shots from the back of "the court", then move forward and hit a couple half-volleys, move back to the back and so on. It improves your backwards-forward movement and that's another thing some probably do to when having their on-court practices.
I decided to end my wall practice with hitting at a slower rate. I think of that as an important practice principle anyhow; even when I go to a fitness I spend the last 3 minutes doing calming-down, yoga of a kind. To get rid of everything that happened in practice and start focusing on new stuff to do. So what I decided to do in the last 5-10 minutes was playing basic shots, that I don't play otherwise. First I played with my left hand and gradually started playing my left handed shots with faster speed. That's a great exercise for your general coordination. You also have to move your legs differently; once again, you're thrown out of your comfort zone. But you enjoy it, cause you know that outside of your comfort zone you learn much more than inside of your comfort zone.
After playing left-handed, I played one-handed backhand (note: I didn't remember to play two-handed forehand, but I'm sure it'd be useful). It's just these small movements during the shots that you learn when repeating them and they will come in handy sooner or later. What I'm trying to say is that probably 95% of your time spent on court you won't need them; but knowing you master them, it will be subconsciously a great boost to your confidence.
It is a very common philosophy in surfing, but I really wanted to carry it on to tennis and to life, cause it should be applied to both of them. Be adaptive and creative, don't hang on to ideas about yourself and the enviroment you live in. Something surprises you, that much better! Remember, those things once fascinated you! Just know that everything is what we make of it and that you can turn every bad thing into a good thing, because you are an organism that naturally strives towards survival.
Oh, and 5.) YOU START THINKING how awesome it would be if you could be that wall, even if for just one match. After you start thinking it, try doing it. Even if your style is agressive ballbasher, the wall teaches you good counter-attack strategies, which come in handy at any point in the match.