Step... Step... Step...
Just kidding. The key to a safe trip up Rainier (IMHO) is knowing glacier travel and crevasse rescue. The Mountaineers cover this stuff for about $400 in their basic class, the Boealps do the same thing for $200. Both of these classes teach you a whole lot more than just glacier travel.
Another route to take is to get a good book (Freedom of the Hills) and practice the techniques in a safe setting. The techniques are not difficult, just specialized. Throw a rope over a tree branch and prusik up. Learn to build snow anchors. Set up a Z-pulley over a large snow embankment and pull someone up.
My last recommendation repeats Blake's thoughts. Get some experience. Climbing Rainier in good weather can be a cakewalk. Climbing Rainier in a nasty storm can be terrifying.
Good luck.