When the party's over, it's time to put the decorations away. If you don't want to deal with helium Mylar balloons hanging around forever, consider deflating them to reuse -- all you need is a drinking straw. Anaheim Public Utilities also recommends deflating balloons before you throw them away, but you can just pierce those bound for the garbage with a knife or scissors.
You can reuse Mylar balloons repeatedly until the self-sealing valve fails.
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Step 1: Insert the Straw
To deflate a Mylar balloon, insert the drinking straw into the balloon tail valve. Push the straw far into the balloon through the valve, until only 1/2 inch of the straw is sticking out of the valve.
Step 2: Depress the Balloon
Press down on the body of the balloon gently to remove the helium. It can be difficult to get all of the gas out of the balloon, but try to get as much as you can.
Step 3: Fold the Balloon
Fold the Mylar balloon for storage. At this point, remove the straw and fold the balloon body. Some gas may remain trapped inside, but you can squeeze most of it out. You may also leave the straw in to remove more gas as you fold. If you choose to do this, fold the top part of the balloon down and press flat. Fold it down again, if there is room, until just above the end of the straw inside the balloon body. Smooth the balloon flat. Fold in the left and right sides of the balloon, pressing helium out as you go. Remove the straw, and fold down the rest of the way.
Leave the ribbon attached to the balloon tail. Wrap it around the folded balloon to keep the folds from coming undone.