There's well enough laid out rules for taking PrCs beyond their usual level cap and advancing their class features in whatever pattern they already do so in. In the DMG I think?
Is there an actual rule anywhere that states you can only take a prestige class once? If there was a little 5 level PrC, you "finish" it at level 10, could you take level 1 all over again when you level up? You still qualify for the PrC after all.
ADDING A SECOND CLASSWhen a character with one class gains a level, he or she may chooseto increase the level of his or her current class or pick up a new classat 1st level. (A character can’t gain 1st level in the same class morethan once, even if this would allow him or her to select differentclass features, such as a different set of domains for a cleric.)