This one actually comes courtesy of my BF and is something I never really thought about doing. It's a good idea if your shoes are in rough shape and you don't have the money right away to get a pair of new ones.
Cheap insoles can be found at the store. At Wal-Mart, you can get a really cheap, standard pair for less than a dollar. Depending on your preferences and how much you have to spend, you may want to spend a little more, but we're going really cheap these days.
Find some heavy cardboard and super glue the insoles to it. Then cut around the shape, cut to size, and fit into your shoes. The cardboard reinforces the insole and covers over any areas where the rubber sole may have started to come through.
This is not a permanent fix. If your shoes are in bad shape, they can cause foot and back problems if you continue to walk on them. However, if you just need something to tide you over until you can get to Payless (or find a cheap pair at a thrift store or garage sale), this is a good solution.