I have always been extremely stingy with magic items (since the BECMI days; carried this habit well into 3e), way below the averages in the 3.0 DMG (actually bordering on the bare minimum), so stacking of bonuses has rarely been problematic. I also use the DMG variant "power ingredients" for the construction of magic items, so it's completely out of the control of the players: to obtain power ingredients, they have to go on adventures, which in turn consume resources. For some magic items, there are no known power components at all (bye bye wands of cure light wounds!) I also use training, which means consuming monetary resources; and if the environment and the trainers are not suitable/available, no multiclassing is possible. But demihumans can multiclass from 1st level (per 3.0 DMG guidelines), although, again per 3.0 DMG, not all demihumans can access all classes (I generally follow the AD&D guidelines for possible combinations.) Finally, no prestige classes (unless they are strongly tied to setting elements, like the Knights of Solamnia; but even then, I generally prefer to work with base, PHB-only, classes.)