Gwen Carbone has been putting together some great little videos lately. In this one she explains how to tie-down a board on your roof racks. Just a couple of little points I would add. Putting your board inside is OK for short, low speed trips to the beach, but I would highly recommend, for safety sake, that for highway trips you put it up on the roof racks. I had a customer come in to the shop who told me about a bad car accident he had, with a fibreglass board inside his car. He still has fibreglass splinters in his head and neck. Even an inflatable board could cause problems inside the car in the event of an accident.
If you need good tie-downs we have some in the shop. We also carry lockable tie downs, in case you are worried about your pride and joy being stolen.