Wow, Speedy - Great thorough and helpful reply!
Obviously "typical" bad habits for beginners and someone who's been skating for 11 years are going to be very different!
I'd say the most common and most power-robbing mistake an experienced skater who has had no formal training would be making is likely to be the infamous "toe flickie" push. (In other words, "not pushing through the heel.) So I'll start there. Virtually every skater does this until it is "trained" out of them.
If your toe wheel wears faster than your other wheels, you're guilty of being a toe-flicker. The good news is fixing this will make a big difference in your speed and efficiency.
My favorite way to drive the toe-flicking demons from a skater's body - so he can be Saved! (Everyone say "Hallelujah!") - is with half swizzles.
First, warm up with some two-foot swizzles. Note: Swizzles are when you propel yourself forward while keeping both feet on the ground and pushing them out away from each other into a wide stance and then drawing them back towards each other into a narrow stance, and repeating.
Make sure you're bending your knees while your feet are close together so you can use your powerful quadriceps muscles in the push. The motion should be a smooth blending of down, out, up; down, out, up. If you're just going in and out, in and out, with no "down," you're leaving out the knee bend and missing all the power.
Okay, as long as you're able to propel yourself along without losing momentum, even on a slight uphill, you're ready to go to the next step: half swizzles.
A half swizzle is when you glide on one foot (the support leg) while the other leg (the action leg) performs its half of the swizzling motion. Start out by doing repeated half swizzles with one foot. Because you are not lifting your foot off the ground, you can't do a toe-flick at the end of your stride. Also, again because the recovery takes place on the ground, you will be forced to begin to turn your toe slightly inwards towards the line of travel at the end of the push, which forces you to push through your heel. Try it and concentrate on feeling the power coming through the heel - imagine a bolt of lightning traveling down your leg and out the heel.
Of course the next step is to switch legs and repeat on the other side so you stay symetrical.
Finally, perform alternating half-swizzles: right, left, right, left, etc. Feel yourself pushing through your heel on each push.
You'll probably notice you're pushing closer to straight out to the side now, instead of diagonally back, an added side-benefit to this exercise, but that's another topic, maybe for later.
Meanwhile, getting back to exorcizing the toe-flickie demons, it's now time to go back to a regular stride where you can lift your feet off the ground at the end of each push. However, your goal is to keep the feeling of pushing through your heel instead of going back to the devilish toe-flickie pushes!
If it helps you understand why this makes a difference, imagine you're standing in front of a burning building and there are kids inside in need of rescue, but the door is locked. Are you going to kick that door in with your toes or with your heel? With your heel, or course! Why? Because you'll get much more power that way.
The same is true of your skating stride. The force created by your strong glutes and quads is transmitted directly through your heel with no power loss. But if your relatively small calf muscle tries to help out the big guys, all it can do is point you toe at the end of the stride causing a weak toe flicking effect. At even a moderate speed, you cannot flick your toe back fast enough to even match the speed at which your foot is rolling along the ground, thus it is actually dragging on the ground (hence the wheel wear!) Worse, your floppy ankle is now absorbing and wasting some of the larger muscles' power
If anyone has some videos or links to videos showing the contrast between a toe-flicking stride and a heel-push stride, or even swizzles and half-swizzles, please share them with us here! My audio-visual department (Super-hubby Pete) is rather tied up these days with his work at Arizona State University and the whole back-to-school thing so it will be a while before I can get out and film anything.
And if toe flicking is not your particular "sin," please feel free to let me know if there's another common error from Speedy's list that you need some help fixing.