If I'm eligible for assignment and something new comes out in my price range that looks interesting, I'll request it. If I'm not due for assignment, I usually debate it for a day or two and see if a sale on it pops up. I usually shop around the sales.
I have a couple of wishlists where I deposit new things. If it's something I absolutely have to have, it goes in the "Things to Buy First" list. If it's cool, but not a pressing need it goes in the "Things to Get (But Not Immediately)" list. It kind of helps me remember what is grouped where.
I do a little mix of everything!! I depend on my wishlists a lot when it comes to making a purchase. If something new comes along, I see if I can grab it for an assignment, if I can't, it gets added to a wishlist for when it's available on here.