Prone to self-sabotage? Even the best-intentioned travelers sometimes undermine their chances of a great vacation. What you do (and don't do) on a plane can make or break the trip to come. Seemingly innocuous activities can set you up for exhaustion, illness, and jet lag—all total vacation killers. Here are eight ways you might be ruining your vacation even before you land, plus tips for beating the odds and arriving primed to squeeze every last fabulous drop out of your travels.