Just wanted to second the idea of spreading composted manure (or other compost) over the grass each year, and mulching your grass clippings. It won't improve the lawn immediately, but over time the soil will get better.
I've also spread rabbit food (the alfalfa pellets) over my lawn in the past. If you buy them in large bags from a feed store, it is a cheap way of getting organic material into the soil. Depending on how many issues you have with rabbits, though, this may not be your best option. I don't have a ton of rabbits here, so what little they ate was more than repaid with bunny droppings, which were probably even better for the soil than the alfalfa pellets.