Mylar balloons, more commonly referred to as foil balloons, are large balloons made from foil as opposed to latex. Unlike latex, which has pores that allow the helium to seep through, mylar balloons hold in the helium for a much longer period of time. Because of this, mylar balloons are generally much more expensive than latex balloons. If you want to get the most for your money, save any old mylar balloons and reinflate them when the next party or special event rolls around.
Locate the small tab where the balloon was originally filled with helium.
Insert a straw up inside the inflation flap until you notice that air begins to escape from the balloon. Press on the balloon to help the air escape through the straw. Be sure to remove all the helium from the balloon so that when you refill it, there is only new helium inside the balloon.
Insert the tip of your helium tank nozzle inside the inflation flap and begin filling the balloon with helium. When you see that the balloon is fully inflated, and there are no creases on the balloon, remove the nozzle from the balloon. Unlike regular latex balloons, mylar balloons seal immediately --- you don't need to tie off the end to prevent air from escaping.
Take the empty mylar balloon to your local party supply store to have it refilled if you don't have your own disposable helium tank or rental tank. The store will charge you the cost of the helium to refill the balloon.