There are optional rules about Guild Training (esoteric training and exoteric training) whereby a spellcaster can re-gain lost levels of spellcasting. Specifically, at about 5th level one spellcasting level can be regained - eg A 5th level wizard could take a level in any class but still count as a 6th level wizard in terms of spells per day, gaining new spell levels, etc - and then at about 13th to 15th level (depending on how many Fame/Prestige Points the GM awards), the character can regain 2 more levels of their primary spellcasting class plus one level of a 2nd spellcasting class.
My question is: which class(es) would you suggest taking for a Wizard? (I like wizards.)
Eg I could take one level of cleric at 6th level, followed by a 2nd level as soon as I'm allowed which would then count as 3 spellcasting levels and qualify for Mystic Theurge without ever dropping any wizard spellcasting. The cleric levels wouldn't ever be high, but having at least 4th level cleric spells is better than not having them!
Is there a better route to follow?
There are a lot of prestige classes that lose spellcasting progression (and so aren't a good idea to take) that become more viable with the Guild Training.