You live in central Florida and say you have sandy soil. I think I know what you're talking about since I live not too far north of you and have sandy soil that is so sun-beaten that it looks like it's devoid of any organic matter, much like an ash tray. However, if you look under my mulched area under the live oak and magnolia trees you'd see a very vibrant commuity of soil organisms and very much loaded with organic matter -- the distance between these two areas is only separated by a driveway.
So I know what you're dealing with, in my sandy sun-beaten area of the yard nothing, but the hardiest of weeds can survive and there's no way I could expect an earthworm to survive in there, even if I digged a hole for him. I can't give you great advice, because I'm still in the process of building soil in this area and I've had some setbacks. However, I've had some sucess on a smaller scale.
I refuse to pay money for soil amendments, but this is basically what I do:
1. Get mulch -- I do this by going around neighborhoods with big oak trees all around. These people leave large bags of leaves on the sidewalk for city disposal. Or, you can contact local landscaping companies, many haul away the leaves and would be more than happy to give away, usually easier than driving to the dump.
2. Compost every thing
You don't need to build a compost bin, you can just throw your kitchen waste in the mulched area that you are trying to build up.
3. start planting easy stuff to grow, such as sugarbaby watermelons and sunflowers. Also look into planting beneficial "weeds", to help build up the soil. BTW, I don't rip the plants up by the roots when they're done producing. I simply cut them up and put them under the mulch so the will become soil.
If you look here
you'll see on area of my yard that I've had a lot of success with, but only because I've spent more time in that one area. My problem is that I don't produce enough kitchen scraps to build up the entire yard (right now at this moment!!!) as I would like, so I gotta work in sections and since I wanted to plant a banana tree and knew they were heavy feeders I concentrated my efforts in that area first (about an 8' x 10' area).
The secret to mulch. Everyone will tell you that you mulch so the soil does not lose the moisture to evaporation, but that's NOT the case in very sandy soil (I know I've tested that before). Very sandy soil, regardless if you have mulch or not will lose the moisture due to drainage and will not retain moisture, like healthy soil does, so mulch or no mulch sandy soil will lose water. However, what the mulch does do is create a microclimate for the organisms that work to make a soil healthy -- they need moisture and they need shelter from the sun.