Seeing as I don't own any pets and don't have kids, it's not an issue of keeping the lids on for the safety of those who don't know what they are doing with a spray can, so I figure I'm justified in making my life a little easier.
A spray can
A pair of scissors
Step 1: Take the lid off
Once the lid is removed, you'll notice a small lip on the inside of the spray can. It is this little lip we are going to focus on.
Simply take the scissors and scrape away a small amount of the plastic around the cap so that the scissors go all the way around the lip of the cap - but careful not to remove it all, because you still want that SNAP of the lid going on and off. We just want to remove enough that the lid is easier to get on and off with your own bare hands.