Mike,
One solution for active ventilation, is the following:
http://www.envirofan.net.au/dual_fans.htm
I didn't buy the product, they wanted something like $240! They're basically just 2 computer fans wired up in parallel, mounted on a plate (eg, aluminium, perspective) then wired up to a 12v regulated power adapter. I built my own due to damp issues under our house and it hasn't missed a beat in 3 yrs running. I have a timer but it runs pretty much from 8am to 8pm. Running costs like $~$3-5 a yr, to make about $40-$50. That said, Jaycar have some nifty solar panel devices and kits and you could probably come up with a solution thay way, given you don't have power to the shed.
Also not a bad idea to lay some plastic under the slabs, moisture will find its way up and also help keep any weeds coming thru.