Reward the dog so that he looks on distractions as an opportunity to earn a big reward instead of fearing them as an occasion to be corrected. Start with the dog in a sitting position and then do a come signal. When that is secure teach a sit from a down, then a come. If the dog looks away and doesn't look back quickly, move closer. You may use any signal you devise, as long as it is one continuous motion.
Some trainers alternate hands for signals.
Some carry two shirts to shows and wear the one that contrasts most with the background wall. The arm should not extend to the side, because it can cause confusion with directed jumping. Down - may be given either with the arm raised overhead and dropped, or by bringing the arm straight up over the head and then returning it to hang by the side. The latter is called a windmill down signal. Whatever you choose, please remember that one continuous motion is required.