When I was still in business, a typical proposal for installing a gravel driveway would look something like this:
Excavate the entire area approx. 8" below final grade.
Roll and compact the subgrade.
Lay 4" of No.3 stone
Lay 4 compacted thickness of CA-6 Limestone.
It is important that the subgrade be fine graded and compacted before adding any base material(a smooth drum roller would be best, but your tractor tires would work also). The finished driveway should be slightly higher than the adjoining landscape so that the water will drain off. As others have mentioned, be sure to do any work along the drive to keep water away. If you run into any soft or wet areas while excavating, you should dig it deeper and add more of the 3"(or larger) stone.
I don't know how big your driveway is and this could be overkill if you have good clay and good drainage in the area.
Also, it wouldn't hurt to rent a vibratory roller after it rains to compact the driveway. Just be sure to go slow with it, maybe 1 or 2 mph.
Good Luck,
Jeff