Most of the time, I find multi-classing most effective at low levels. Having that one level in sorcerer so you can use scrolls and wands and cast a few buff spells is really handy for a Sorcerer 1/Fighter-type 1. The ability to use wands and scrolls remains very helpful throughout a character's career.
Likewise, a level in cleric is very good at low levels for the healing.
But, I usually leave the spell-casting class at only 1 level or so. Those additional levels don't add enough bang for the buck unless you concentrate on them to stay up to speed with the rest of the party.
My favorite multi-classing always involves the rogue, however. A rogue/fighter is my favorite. Fighter for feats, and rogue for skills/sneak attack is a nasty combo. Always max out your Use Magic Device and you are a pseudo-spellcaster, whipping out scrolls and wands. Rogue/fighter stays effective through all levels.
My next multi-class will be a rogue/ninja. How does +2d6 ~sneak attack at 2nd level sound? Or a rogue 1/ Ninja 2, with ~sneak attack +2d6 and invisibility for 6 rounds a day (assuming 18 wis).