An addition to the plywood idea. One that i used was to cut strips of ply about 2' wide at full 8' length. Put the ramps at the far end and use little strips of plywood screwed down to hold it in place. By the time your front wheels get to the ramp hopefully your back wheels will be on the ply and the ramp can't go anywhere.
In my dad's shop he drilled two 1/2" holes for some 3/8" cement anchors. He bolted two pieces of 2x4 to the floor. For FWD, he puts the ramps on the far side of the 2x4 so they can't get sucked back under, and for RWD he puts them behind the 2x4 so they can't get pushed away.
On RWD I've also seen guys tie a rope to the ramp and string it back under the rear tires. As they go up, the rope holds them in place. Otherwise, good luck. I ended up building some wooden ramps. I took two 10' 2x10s, cut them into 4', 3', 2', and 1' lengths and screwed them together in steps. Its solid so it can't collapse, and your rise is only 1.5" every foot, so its easy to ramp. Of course, it only gets you up 6", but its better than nothing. You could buy more pieces and make more steps, but I'd be tempted to jump to 2x12 and for the ultimate, get some rough-cut oak from a local mill for supreme sturdiness.