At the workshop, we used something called "blend". It's... a blend (as you probably guessed) of PVA and paste from purified wheat starch. PVA is not 'archival', which means that it contains acids which may damage paper in the long run, but starch is pretty sticky and is acid-free. (On the other hand, starch is also the favourite food of the silverfish, so I'm not so sure you want to have a whole archive with starch-containing covers, but that's a different thing.)
Anyway, the starch paste dries out pretty slowly, which means that the starch keeps the PVA quite supple. I had ordered some purified starch from a bookbinding webshop, and my book on bookbinding contained a recipe to make the paste using a microwave (so it only takes three minutes instead of ten).
I've been using the blend today, and it's indeed much, much better. Tomorrow evening, I'll finish up the box. Of course, there will be pictures when it's finished!