We've had a few big dogs (60-190lbs) - throwing a treat in the car and having them "go get it" can motivate them to get in themselves. If they get to a point where they just can't do it (arthritis, etc.), you can have them put their front paws in, then give them a "boost" for the rest of the way.
Other than that, I'd go with some portable steps or a ramp. Sometimes a side door vs. the back hatch works easier for climbing in, too.
We were able to get away with treats and the occasional "boost" for ours. Haven't used a ramp... yet...