I would agree with Slazenger07 in that you need to be comfortable hitting your 1HBH with topspin as well as slice because you will eventually have players good enough to poach aggressively when you slice it. It will eventually become a liability if you can only slice it.
As far as how to hit it, here are a few tips. First thing I would suggest is patience. I know it sounds counter intuitive but the biggest problem most people have is they panic and try to muscle the ball thinking they have less time than they really do. They panic and rush the shot hoping to save the day with some wrist manipulation. This may work up to a certain level but will hurt you if you rely on it especially against bigger servers. The first thing to remember is you have much more time than you think, even against big serves. To prove this to yourself, check the you tube video of Fed returning a 140mph+ serve from Roddick. Notice in the vid how far he takes it back on a serve that hard. My point is when you are facing the 80-100 mph serve of a 4.0, you have plenty of time IF you don’t rush it or panic.
This shot is all timing and when hit cleanly, you will be surprised how you don’t even feel a major jarring impact even against very big serves (like a drive of the golf ball on the screws so to speak). The first key is to keep a firm wrist. You want it locked into position. Try to keep a constant angle/relationship between your wrist and your forearm. Another thing to be conscious of is trying to hit it too far out in front of you (again, this is usually because you rushed the stroke, thinking you had less time than you really did IMHO). It stands to reason it also hurts your extension through impact if you have already extended to early. Try not to rely on using wrist manipulation to save the shot, as that will hurt you in the long run (also caused by rushing because again, you are too early). Next try to extend through impact, especially against the serves you are facing. This is very important and it is tough to do when you shortened the stroke too much and are already at full extension and thus can get anything on the shot (I hope you see the theme about the problems of being to early/panicking/rushing the stroke).
Also consider your grip and whether you change to hit the 1HBH return or wait with the BH grip. I personally wait in the forehand grip and change to the 1HBH grip when taking it back. As per the theme above, if I rush the shot and don’t take a full backswing (speaking relative here) I don’t get my grip turned all the way to my standard 1HBH grip. Obviously a problem, also caused by rushing the stroke so be wary of this. And for a double whammy, if you don’t get the grip fully turned, your contact point is slighty farther back than normal (again causing you to be early) if you try to hit it with the normal contact point which doesn’t work unless you use the wrist to try to save it (again, bad idea but admittedly does happen occasionally).
Make sure you keep your back arm behind you to keep from opening your shoulders too soon. This also can cause many problems and should be something to keep an eye on. You don’t need to move them as you can get all the pace and spin you need extending through impact with your arm and counter weight with your firm wrist. At that serve speed, you have time to get sideways on the return which will also help you hit the return more like a normal 1HBH shot, making you more comfortable with it. It should come together like this: split step at contact, recognize coming to your BH side step behind with and turn left foot (assuming righty and you have time), unit turn back (grip chang e on the way back if applicable, shoulders turn sideways, off hand (left hand) still on racquet to help facilitate shoulder turn, no need to lengthen backswing any more than this shoulder turn and automatic coiling from the shoulder turn), step into shot pushing off left foot (if time and you have time against the serves you are talking about), extend to your contact point with a firm wrist at contact, left arm staying behind as a counter weight and to keep shoulders opening (even more important for a righty returning a serve from the deuce side), extend through impact, keep head down and don’t pull head up to peek, follow through and then admire your great shot (hey, I am an optimist). This is a rough example of the things happening but all steps may not be applicable depending on many factors such as serve speed and so on.
I also find that this is a confidence shot. If you have confidence you can hit it cleanly, you can and do. But when you start doubting it, your head gets in the way and it can go downhill fast (and your opponent will notice and feed you a steady diet of them or I would). In my opinion and experience, I miss many more returns from being too early than being too late.
To practice, have someone hit serves to your BH, wait in a BH grip to remove one variable if you want, work on the foot work (as usual important especially against slower serves or when you have to adjust to put yourself in position) and then it is all repetition and gaining confidence in your stroke.
Best of luck. Much easier to demonstrate on the court than explain out loud.
TM