we did the following from 3.0 through 3.5. The game never derailed and everyone always had a great time. It worked fine from 1st level through 20th level and i lost count of how many characters made that journey through 20 levels. I have posted this in several threads in the past and there were always those negative nellies who would say “but that means a 15th level fighter could fall to a charm person spell!" we always thought “why shouldn’t a 15th level fighter be afraid of falling to a charm person spell from a 20th level wizard?”
the dc for spells is 10 + ½ the caster’s level + relevant ability bonus. The dc of a spell from a 20th level wizard with a 20 int would be 25.
For multiclass casters where all classes are of the spell-casting variety, the dcs worked as if they were single classed…so a 10/10 wizard cleric with a 20 int and an 18 wis would have a dc of 25 against his arcane spells and a dc 24 against his divine spells.
Multiclassing with a martial class gave you 1 level for every 2 levels of the martial class (by martial class we included everything but cleric, druid and wizard)…so a 10/10 fighter/wizard with an 18 int would have a dc 21 for his spells (10 + 2 for fighter +5 for caster + 4 for int).
The “penalty” if you will in such cases is that the caster never gets a spell higher than 5th level (assuming he stops at 10th level). “dc 21!!!” you might say “the fighter cannot make a reflex save that high at 15 level!!!) yeah, so?! That 30 or so points (maybe) of damage he is going to take is going to make a big impression on that 15th level fighter…yeah, right.
We also used the same principal for sr and it worked just as well.