The best thing you can do is add lots of organic matter to it. If you have a good source of compost, that would be a good thing to add. I would stay away from using a lot of manure because it can add salt and that can be a problem around here. Leaves are fine to pile on top but should only be mixed in the soil if they're composted first. If you have a Starbucks near you, they give their used grounds away for free (you don't need to buy anything, you're saving them money on trash removal). The coffee grounds can be mixed into the soil and are a mild fertilizer as well as adding organic matter. You can add a lot if you mix them in with the soil, but if you put them on top, don't make a layer more than about 1/2 inch thick or they can crust up and repel water. You can also mix them with leaves to help create compost.

If it's really clay heavy, you could mix in some sand to give it some water holding abilities... Other than that, just a bunch of organic stuff.

I agree with the posters above on improving you soil.
A quicker, more expensive route is to build a raised bed. Then you can plant in it, the same day you add the soil mix to it.

"If it's really clay heavy, you could mix in some sand to give it some water holding abilities"

Organic matter is temporary and needs to be added every year. Sand is permanent.

The problem with sand is that it takes a lot to help. If you don't add enough, you can actually make it much worse. The extension service recommends against sand for that reason. If you add too little sand, it will set up like concrete (technically more like adobe, but it will be rock hard and much worse than what you have now).

If you want to add sand, just make sure you add enough. Also, make sure it's coarse sand, not playground sand.

I did a blog post on this very subject since we had to do the same thing.

http://www.vegenag.com/2009/03/expanding-the-garden/

Basically you want to add sand and peat moss to help the texture and a balanced fertilizer to help the soil content and pH. Continue to improve the soil every year by adding organics like grass clippings, dead vege plants etc.

For more information on the fertilizer go to:

http://www.vegenag.com/2009/03/fertilizer-basics/

Here is a link that might be useful: My Blog

I know this is late in coming, but its never too late to amend your soil. Perilite is very cheap and can really loosen heavy soil. I personally put steer mulch in once or twice (depending on the soil)in late fall. This breaks down more slowly and also helps keep the soil loose. Most of the wood is sufficiently composted that it is in soil form. The bigger pieces take longer to break down which continues to provide nutrients over time. From there its organic compost. Being that I'm lazy, I use osmocote fertilizer so that I can put it down once and be done.

I had clay soil as well. I add peat moss and ut-lite to each bed. The peat moss helps to acidify the soil and the ut-lite breaks up the clay.