Mylar balloons have colours and prints that just can't be matched by traditional latex balloons. Mylar balloons are made of flat sheets of mylar and normally have wrinkles even when fully inflated. Balloons under 18 inches are often permanently heat-sealed, but larger balloons have one-way valves that can be opened to deflate the mylar balloon.
The One-Way Valve
Most mylar balloons have an internal one-way valve---you can blow them up, but you can't deflate them. The valve is a few inches up the stem of the balloon. This makes it hard to blow up the balloon by mouth. To open the one-way valve, gently push a drinking straw up the stem until you feel resistance---that's the valve. Push slightly past, and you have an airway into the balloon.
Inflating a Mylar Balloon
Use your fingers to seal the mylar around the straw and inflate by mouth. If you have an air pump----a bicycle pump or foot pump to inflate pool floats or inflatable boats---you don't need the straw. Just insert the nozzle into the stem and seal the mylar around it with a rubber band, masking tape or your fingers. The pressure from an air pump will automatically open the one-way valve as you inflate it.
How Much Pressure to Add
Mylar is different than latex or rubber---it is a low-stretch material. Once the basic shape is inflated, it can't be made larger by adding more air. Large wrinkles can be removed with more air pressure, but small, tight wrinkles are normal when the balloon is properly inflated. Commercial pressure valves cut off at one p.s.i. (pounds per square inch).