Compost or composted manure will do it. Check out the Soil, Compost and Mulch forum they can really help you out with any specifics there. The basic answer though is compost (aka humus).

Agree with the above. I add compost, dehydrated chicken manure and lime (if needed). Also, urea might be a good additive to add nitrogen to your soil.

A good idea is to get your soil tested or measure the pH with a pH meter because nutrients cannot be accessed by the plant unless the pH is at an optimum level. Throughout the growing season, I add fish emulsion, too.

compost! And since you have a tiller it's easy to till in autumn leaves in the fall.... instead of dragging them to the compost pile.

....any manure and/or (duh) chemical fertilizer.

Thanks for all the great suggestions.

Gardenguy- I just read my post-
I hope the 'duh' didn't sound insulting, it was supposed to be a "I might be stupid for even making the fertilizer suggestion, I'm sure you already thought of it" way.

sorry, some thoughts don't always come across well in writing!