I also have a background in usability and HCI and have applied it liberally during the design and testing phases of my games.
Since my current project is on a much larger scale than my previous games, I'm making sure to have more eyes on the game as I release occasional alpha versions for play testing. I currently have two people outside of the main team who have been looking at the versions and giving feedback. I ask some specific questions to make sure that the interface is intuitive and easy and make sure that the puzzles make sense, and then also ask more broad questions about characterization, dialog, graphics, etc. I've gotten some good feedback and have made some tweaks here and there that will hopefully improve the overall experience.
As far as prototyping puzzles to make sure they're fun/playable, I did a bit of that with Linus. You can play a flash version of the slider-puzzle-ish thing in this
post mortem article on my blog. Though really, I was prototyping to make sure it wasn't too difficult (which actually led to the broken panel on the right to easy it up a bit).
I also, of course, have a long period of beta testing when production is complete to iron out any bugs that may have somehow slipped through.
