The client usually has a good idea of who their customers are, and I find out as much as possible from related or competitor sites and marketing materials. I imagine myself as the customer, and think of how I'd like to proceed through the site to find various information. One of my favorite places of inspiration is in a mall. I don't just love to shop, but just about everything you see is well designed. From the logos, the displays, the directories, to the products themselves.
Grocery stores are just as cool.
I can spend hours wandering the aisles and looking at packaging. Personally, I purchase for the package design more than what's in it. Inspiration can be found anywhere, because design is everywhere. Whenever I feel completely out of ideas, a good hard play with my puppies usually cures me. I think it's just a way of clearing my mind.