Next, just put the bag in the dryer. There is some debate on how best to do this. One school of thought: Add a clean tennis ball or two to the dryer, as this will break up the down clumps. Some argue that this can damage the baffles. Myself, I'd go with the tennis balls. Run the dryer until the bag is thoroughly dry, but watch the heat setting so you don't melt the shell. Then, grab the bag by the foot and give it a good, long shake. This will help re-distribute the down through the baffle channels, with luck eliminating the thin spots you're seeing.
And that should do it. As a matter of course, I recommend always wearing something to bed -- a cotton T-shirt and light long underwear is fine. That helps keep the bag much, much cleaner, extending the time between washings and extending the life of the bag.