In reply to TheFasting:
As Phil (Rocksol) said, get comfortable at big mixed ED1 in a variety of conditions and weather.
Do Big stuff in the Alps.
Do Big stuff in the Alps in winter.
Do other routes in Alaska - some people actually do the West Buttress of Denali as preparation for the Cassin.
Have plenty of time and patience regarding the weather (I've spent several weeks over 2 trips there waiting for half decent weather on Denali and still crapped out on both occasions.
The two Tom's on here (Livingstone & Ripley) and also Will Sim, made ascents of the Cassin when they were only about 21 - although they are all rather talented lads - and it does get guided up on a regular basis.
However, it's a big serious brute of a hill, and bad weather in Alaska is in a completely different league.