How do I properly store board games long term?

I guess it depends if your garage is insulated or not, and the kind of temperatures & humidity levels you can expect in the winter/summer.

If you live in a dryer climate, the games can be stored in a Rubbermaid tote with snap on lid out in the garage. Be sure to separate the boxes with sheets of craft paper or butcher paper, which will help absorb any moisture and prevent the boxes from rubbing against each other.

If you live in a humid region, the games need to be stored indoors or they will get mildewy. While you can still store them in a tote, the simplest solution is to stack them in a high cupboard or shelf where they are out of the way.

Put them in a large plastic bag, tie it shut. If there's any question of dampness, get some silica gel and put it in the bag too. Plastic boxes don't seal well enough to keep dampness out.

They need to stay dry, mostly. Put your pile of boxes into a bag or box, to stop dust from piling up on the games. Mine are stuck in a dry basement closet.