The majority of balloons are made from latex or foil, both of which are prone to oxidation over time. The oxidation process, where the latex or foil slowly degrades and the balloon sags, is not entirely stoppable. You can, however keep balloons from oxidation for a period of time.
Squirt a tiny amount of Hi-Float into the balloon and move the product around the entire inside by squishing the exterior of the balloon between your fingers. Inflate with helium as usual. Hi-Float is used for helium-filled balloons to keep the balloons from oxidising and make them float longer; it can't be used with non-helium-filled balloons. Inflate non-helium balloons with air.
Put all the balloons into garbage bags and seal the bags tight. This stops fresh air from getting to the balloons and slows down the oxidation process considerably.
Take the balloons to the location where they will be set up while still in the garbage bags. If the balloons are to be handled, such as balloons on strings at kids' parties, then the plastic bag is the only means of keeping them from oxidation; store in the bag as long as possible before the party.
Set up balloons intended for static displays like balloon arches or centrepieces that will not be touched or handled after being put on display. Spray the balloons with hairspray or a silicone spray. This keeps the air from breaking down the latex, but if you touch the balloons after spraying, they will wrinkle.
Balloons inflated and stored in plastic bags will keep well overnight. Mattress bags are ideal for large numbers of balloons.