There's a tire sealer called "Slime" that works like a charm. On my lawn tractor, the sidewalls on the tires were horribly cracked and the tires were obviously shot. On a whim, I bought a bottle of "Slime" and poured it in according to the instructions, and the horrible tires actually lasted another season with no trouble!

Slime is a green, gooey substance, and some of it actually seeped out of cracks in the sidewall, but it quickly sealed the tires. I've since used it to fix leaky tires on a wheelbarrow, hand truck, and, most impressively, on my Ford 640 tractor. Works like a charm. Which is good, because I used to have endless trouble with slow leaks.

On Slime Web site, they claim that the stuff never hardens and can be washed out with a hose if you want to repair the tire for real. I haven't tried this, but I believe it.

There are several variants of Slime: for tubeless tires, tube tires, low-speed tires, car tires. You can even buy bicycle inner tubes with Slime pre-installed.

You can find this stuff in hardware and automotive stores, and online.