I use the recommended amount in containers, for the most part. For plants that really love moisture, I put an extra measure into the lower part of the pot. You don't want to mix a double amount evenly into the potting mix for your container; you'll just end up seeing a layer of "jello cubes" that have worked their way to the top after a good rain!
I'm pretty used to working with the crystals, so I usually add them dry to my potting mix (2-3 Tablespoons per 5 gallons of potting mix). When the mix is moistened, of course, the crystals do expand a little. If you're new to using them, it can be helpful to hydrate them first, so you have a better feel for the amount you're adding.
I wrote a DG article on polymer moisture crystals, back in 2008.
http://davesgarden.com/guides/...
