Load Range E tire pressure: how do you keep them up?

I carry a 12v compressor, but it takes forever to add much air. I also carry a Harbor Freight pancake 100 psi compressor ($39.95 on sale). Not very high cfm, but works fine for adding air.

But about the only time I ever add air is at the beginning of each new season. Usually add 2-4 lbs and am good the rest of the year on my Michelins.