The best way is to wait until they are pretty well established (couple full sets of leaves) then start to gradually harden them off by turning off the mat for a few hours then turning it back on... leaving it off for longer each time.. over the course of a few days the plant will be accustomed to normal temps without a heating pad
well just like w3rds says, heat mat is something you dont really need at all, assuming you have decent temperatures where you have the plants. I havent been using heat mats at all and has been working great without it.