I topdressed with compost once and loved the results. (It is a lot of work, BTW).

I did mine in the fall, however, everything I've ever read suggests that you can topdress with compost any time.

One cubic yard per 800 or so square feet is the number I usually hear. If you put too much any time of year, or don't spread it well, you can smoother your lawn. Outside of that, I think the sooner the better generally.

Congratulations on your decision to "go organic". Yes, you can add compost to your lawn now. In my opinion, compost is really really good for your lawn. I make and use a lot of it in my garden, flower beds, and on problem areas of my lawn. That being said, unless you have a cheap source, you may find spreading compost at the rate of one cubic yard per thousand square feet of lawn a bit expensive not to mention the amount of labor required to spread by shovel, then rake or broom into your lawn. There are many things that you can do to improve your lawn without the expense of compost. Cut your lawn high. Water deeply but infrequently. Fertilize organically at least five times a growing season and a heavy fall feeding. Adding organic material to you lawnÂs soil is the very good for whatever grows there. Compost is one method. Mulch mowing all your Autumn leaves into you your lawn is another. Have you started a compost heap ?
Bill Hill

Here is a link that might be useful: Organic Lawncare FAQ

Bill thanks for the advice. yes I have started a compost heap. Its small but I guess all heaps started small. So what you are saying is even though I have hardpan clay for soil I can still build it up without buying compost?

There is some controversy as to the cost effectiveness of purchasing and spreading compost across an entire lawn. I didn't realize your soil was hardpan clay. I would recommend using compost as a top dressing to condition your soil. Organic feedings (grains) alone will have some effect and slowly condition your soil but compost will do it faster and better. As rcnaylor attested, he, myself, and many others have used compost on their lawns with spectacular results. Don't forget the free organic material from composting in place your autumn leaves after mulch mowing them. That helps a lot and is dirt cheap.
Bill Hill

Here is a link that might be useful: BillHill's page