Extreme temperatures are wine's worst enemy, so it's not a good idea to store bottles you buy in the car while you visit other vineyards. If you're near your hotel, drop them off before making another stop. Otherwise, ask the winery to hold the bottles until you can swing by at the end of the day to pick them up. If you must leave wine in the car, park in the shade and roll the windows down a crack, and keep the bottles out of direct sunlight by wrapping them in a jacket or blanket on the backseat or in a foot well. Never put them in the trunk.
If you're traveling by plane, your best bet is to have cases shipped home from the winery, what with all the new restrictions about carrying on liquids. Regulations vary by state, so call ahead to make sure wineries are allowed to ship to where you live. If they can't--or if you really want to bring the wine back yourself--ask the winery for a box and packing materials, and check the case with your luggage.