Some days I just accept that if he falls asleep in the am, the afternoon nap is going to be very late and short. (Like when we have to travel an hour for a doctor's appointment)
Other days I do my best to keep him up, including not going out if I know he's having a rough morning. Or I give him a snack in the car, or toys to keep him busy. Our trip time is pretty quick. I like to do an errand or two a day, instead of packing them all into one morning. Makes me tired too.
This, like all else, gets much much easier as time goes on. Max extended his one nap to 3 hrs most days (from 1.5 hrs to start).