Cover up. Blanketing the popover pan with aluminum foil during preheating will prevent the clarified butter from splattering.
Skip the cooking spray. Nonstick cooking sprays are convenient, but they can also negatively impact flavor and texture (ditto vegetable oil). Use clarified butter.
Keep it shut. Popovers are steam-driven, and allowing the oven's heat to escape may cause deflation, especially early in the baking process.
Fill them up. If you run out of batter, fill any remaining empty cups with water.
Invest wisely. Custard cups and muffin tins are OK, but they don't give the distinctive hot-air balloon shape that comes from a popover pan's deep, narrow cups. Expect to pay $20 for a decent six-cup, nonstick popover pan.
Take a break. Resting the batter for a half-hour after mixing allows the flour's glutens to relax a bit. Just don't make it any further in advance, and forget about freezing it; this is a fresh batter.