I run a dehumidifier in my much smaller 2 car garage here in the DC area. Huge difference in keeping the moisture down.
I would remove any loose items and put them in a pile in the middle of the garage and start at one end and work around to get rid of the loose dirt/dust.
Those three shelving units next the garbage can are candidates for removal. Too much waisted space between them. There should be one long shelving unit. I like the chrome wire shelving at Costco that has 6 shelves each. 6 is too many for my spacing requirements. Since you can make a pole common to to adjacent shelves, extra poles are collected. Three units actually allow you to have a fourth for free with the extra shelves and poles from the other three.
This is a bit of a mess but here is an example:
[IMG]
P1020537, on Flickr[/IMG]