First, get thee to a post office and stock up on "Forever" stamps. They cost $.41 until Monday (when postage rates go up). "Forever" stamps though, can be used for first class postage forever. Even if the rate increases, if you use the "forever" stamp, no need to add extra, it's covered.
Did you know that stamps don't expire? If you find your childhood U.S. stamp collection, you can still use those $.15 or $.19 stamps as legal postage today. You'll probably get more value out of them that way.
Some folks have gotten good deals on stamps on eBay. I haven't tried it yet; I stocked up on "forever" stamps and should be good for at least one more postage increase.