Disclaimer: I've never tried this (but have heard from reliable sources) so don't know if it works and if it does, I think its kinda cheesy but....
If you don't mind switching/abusing earlier versions, it is possible in 1.09 and earlier for characters to get the Ancient exp in town. Therefore, if a partied member (who hasn't killed the ancients) kills them in all three difficulties (norm, NM and Hell) the combined xp bonus should get the charater to approx lvl 55 without them having to kill anything. This could be particularly useful if you are also HF rushing - just take the characters a little bit further in 1.07-9. Of course, you would need to be able to run multiple copies of D2.
If you don't fancy doing this every time, it would be a great way to get an enchantress as mentioned earlier in the thread (or a BO Barb for buffing). This does of course mean that you won't get the Hell ancients xp later in the game (lvl 70+) when it is really handy.
As a sidenote for the enchantress buffing - I've always wanted to try that with a Fishymancer - 12 skellies wandering around doing 2k fire dmg..... drool