I always seem to have problems with my paint thickening in the pot. So, when I came across this excellent blog on extending the life of your paints by Tinweasel, I just had to give it ago.
Unlike Tinweasel, I had no problems finding cheap glass beads thanks to ebay, I got 120 for £4 with postage. My beads arrived this morning and I immediately gave them a go. They work like a treat, a couple of drops of water and a glass bead, a good shake and they're like new.
I'd highly recommend reading Tinweasel's blog and giving it a go for yourself.