[Tennessee Ernie Ford]

All casters improve skills at the same rate (all else equal) but you have to be more judicious in practicing.

Spells that belong to two or more schools only improves one school at at time--the school that already has the highest level. So if you cast 1000 L7 lights, you'll improve your Wiz skill a bunch, but not Alk or Pri.

Erydian's suggestions are excellent. Another choice is to specialization until you get proficient in a desired spell and then make the switch. My priest switched around L14, so that Heal All was robust and then focused on Alchemy. After another bunch of levels, he became a straight alchemist. Given that I finished at L22-25, I would make the first switch at L8-9 playing the same party over again (esp. since the Bard got Heal All around the same time) and the second switch perhaps at L14.

My suggestion won't give you as versatile a PC as Erydian's, but I didn't need a jack of all trades. I needed someone who had Heal All/Rest All and good offense/disabling spells.