I use average hp-per-HD-but-max-at-1st, as listed in the DMG, with one minor change; at even levels, you get an even number, and at odd levels you get an odd number...so;
D8 gets 8 at 1st, 4 at 2nd, 5 at 3rd, 4 at 4th, etc.
d10 gets 10 at 1st, 6 at 2nd, 5 at 3rd, 6 at 4th, etc.
And so on.