This might be too late, but I kept looking at the design forums as they came out and I didn't see one just for general class design, so I'm going to put this suggestion here.
Fractional Base Attack Bonuses and Base Save Bonuses, as per Unearthed Arcana. This rule is already in existence in the case of a PC staying in a single class, so why not for multiclassed PCs? I can understand making remaining single-classed attractive, but it shouldn't be at the expense of multiclassing.
Suggestion: Each class is assigned a type of progression for its BAB and BSBs (Fort, Ref, and Will).
Good BAB: 1 per level
Medium BAB: 3 per 4 levels
Poor BAB: 1 per 2 levels
Good Save: 1 per 3 levels
Poor Save: 1 per 2 levels
Additionally, when you take a level in a class and it is your first class level, you get a +2 bonus to the saves where you have a good save progression.
You then add up all the levels of which kind of progression in each of the four progressions (BAB, Fort, Ref, and Will) and that's your bonus.
This keeps things fair between MCing and Single-classing, and it has the added benefit of reducing the absurdly high save bonuses that result from excessive MCing.