As far as my experience of bouncing with TTL, I usually up the power to compensate and it works fairly well and give a consistant result. I don't do it too often though, so I don't have a lot of experience with that.
Let me add to that a little... a nice beginner's trick to do is one day, when you are practicing, is to set down some base info to help get you into the approximate range, that way you are not starting from scratch all the time, AND if you want to continue to use manual flash settings.
The trick is to recreate and confirm the basic setup that you use most often. Say for example, that you mostly shoot full body portraits and you settle in on ISO 200, flash at 1/8th, shutter speed at 200 and aperture of F/7.1 and when the subject is 10 feet away from you with your camera mounted flash, you get a properly exposed picture. Well a lot of the hard work is now done... take this info (SS 200, ISO 200, F/7.1, 10 ft.) and put it on a small piece of tape and slap it on the side of your flash.
That way, then next time you fall into this situation, you already start off from known good settings. If you are off a little (distance changes for example), you can easily compensate by using a little common sense (ie: either open up the aperture or increase the flash), based on your needs.
Makes "getting there" a little faster.