As far as I know, there are no specific drills that will help you improve the "lens-shots" (I think they're called that in English). It'll come naturally as you spend more and more hours on the court. However, some exercises can help you more than others.
One player plays in front of the T, and the other one behind it. Of course, both must always try to get back to the T after making a shot. The back-court player makes boasts (shots against the side wall from the back corner, aimed at the opposite side front corner) and the front-court player returns the ball with a straight lens. This is a good exercise for developing your straight shots from the front wall to the rear. It can also be done with 3 players (usually brings a better rythm as well) by having one player in the front and two in the rear (one on each side).
(2) "The German"
Can be played by any number of players, but only two will be active. The players line up as usual for the serve, but as soon as the serve is done the quarter that the ball was served from becomes a "dead zone". Thus, you will only use three quarters of the court, and a lot of lens-shots will have to be made. Boasts are another option, when really pressed in the back corner.
(3) One quarter long ball
I just made the name up, but as it suggests - you only use one of the back quarters for playing. All balls must be hit from, and end up in, that quarter (although it is wise to keep on playing even if a ball ends up somewhere else). Variations can be made, such as switching quarter after a number of successful shots.
(4) Going solo!
Simply just play the ball back-and-forth by yourself, and try to make it stick to the wall. The downside of this exercise is that you wont have time to move back to the T, thus not making it very match-like. However, it's always good to be able to practise the fundamentals, without having to run around like a headless chicken (which is often the case when I'm playing!). After all, one has to be able to do it without the pressure of a match situation before one will be able to do it in a sharp situation!
Good luck, and hang in there!