In florida our 'soil' is often not much more than pure sand with a bit of black organic matter dust in it. The absolute best that you can do for it is apply obscene amounts of organic matter such as leaves, mulch(I like to take peoples old faded half rotten mulch when I can get it because they are putting down new), straw, and the detritius from nearly rotten wood/logs. I add hot fresh manure to this to help it start composting, and offset the nitrogen deffiency.
Beware of applying any sort of clay to very sandy soil... It basically mixed and forms something near to concrete.
If you have added a TON of organic matter, and the sand ratio is well decreased, then adding a limited amount of clay(I have used plain, old fashioned, unscented cat litter) can be safely added.