Is there any way to make helium balloons last longer?

Factual Questions
1

We have some at the store to attract attention, and they seem to be worthless after a day, still big but no longer floating up.

I think some sort of spray might help, like Pam or hairspray, to seal off the small holes that must be letting out the gas. But I’m shy about running experiments in front of the others.

2

Also, how can they be seemingly the same size and not float? Does air get in when the helium gets out? How would that happen, under pressure and all.