Here's the method I developed for my current, high-powered campaign. The player rolls two or more dice at each level gain, based on what kind of die his character gets at that level.
d12 = d12, d10, d8, take highest, minimum of 6
d10 = d10, d8, d6, take highest, minimum of 5
d8 = d8, d6, take highest, minimum of 4
d6 = d6, d4, take highest, minimum of 3
d4 = d4, d3, take highest, minimum of 2
I did an Excel spreadsheet of this once to calculate the average for each die type given the unusual rolls. I think it came out to pretty close to 75% of the value of the die in each case.
My players are pretty much all into power-gaming, so I gave them 40-points to buy their characters and use higher-than-average hit points. Then again, all the monsters have the elite array (or better), and they all use the same system for hit points.
Dave