IMO, Alistar's W/Q combo is an example of bad game design. Very few Alistar guides mention it, and the only way you're going to know about it is by watching a 2000+ Elo player stream Alistar games (since they're the only ones that do so). And even then you won't know how it's actually done. Intuitively, it would be done by pressing Q after W, but if that's all you know, you will run into frustration very fast.
I remember talking to some people who said they were having trouble with this combo, so I decided to investigate how it was done by playing against bots and my buddies and practicing over and over again until I could consistently do it. After getting this combo down, I got 7 kills in a 2v2 lane one game.
It's worth learning. Here's how it's done.
1) Pick Alistar
This should hopefully be obvious. But most LoL players are horrible, so you never know...
2) Make sure you can/should do the combo
The purpose of using W/Q is a burst nuke that happens to stun the opponent. Usually Flash/Q/W will be more useful to your team in fights. If you can W opponents into walls, it's more reliable and effective than doing W/Qs. Make sure you have the mana to do it. Finally, keep in mind that if you mess it up, you just blew your cooldowns and made yourself useless for a time.
3) Do the combo
This is the part you guys are interested in I guess.
-Attempt to position yourself at max Headbutt range. It works below it, but it's harder to time the animations.
-Use Headbutt. Make sure you use Headbutt first. It's easier if the enemy is inside Headbutt range when you click, because then there is no delay until the animation starts.
-This is the tricky part: while in mid-animation, press Q. You only need to click it once, but there's no penalty for mashing it.
-The end result should be you arriving at the opponent and stunning him.
-If you pressed Q too early, Alistar will use Q and not W. If you pressed too late, Alistar will Headbutt the champ away and Pulverize thin air.
-If you pressed it exactly, the opponent, when Pulverized, will seem to move towards you as they get stunned.
4) Address your mistakes.
-If you're hitting Q too early, it's because you're probably not watching your W's range, and clicking on the target. This makes it a lot harder. Watch the range of your W, and only use it when opponents are inside the circle but near the edge. Don't just be lazy and click!
-If you're hitting Q too late, work on sliding your pinky finger over to Q shortly after pressing W, as long as you're pressing W properly as described above. Just concentrate on a fast slide rather than pressing Q after W as you would do in a spell combo.
You could play Malphite, but if you learn this combo you basically get his ult on a 8 second cooldown at 40% CDR. Worth it? I think so.