What's under the vapor barrier/insulation is something to consider, too. You'll want no organic material (tree stumps) or loose fill. I use either sand or 304 limestone and rent a power tamper to work it down. Try to get at least a couple inches uniform thickness, more is Ok but too much is wasteful. Then vapor barrier, 2 inch foam and 4 inch square welded wire mesh.
Pound pieces of re-bar into the soil in a few places (out in the middle of the pour) to indicate your finish elevation. Mark either a chalk snap-line or pencil onto the well-braced perimeter form where the elevation is. Pound a few nails along this to help you find it in the heat of battle. You can yank these later. If the pad will be rising and falling due to frost, nail homesote or equivalent around the perimeter to give the slab something to slide on. Have a garden rake handy to pull the wire mesh up into the center of the slab before final screeding but be careful not to snag the insulation. Or just yank it up with your fingers, wear heavy rubber gloves, not those ***** cleaning lady gloves.
After the pour, maybe 24-36 hours, I saw cut control joints every ten feet or so, not for expansion but to make the cracks go where I want them. If you really want to get fancy, fill the kerfs with exterior-grade silicone caulk.
