Here's a thought: Go over your villain's plan and see if there is a spot in that plan where something could go wrong. Also, sometimes it takes an unexpected action from the good guys to exploit that. One of my stories had such an issue and I found that the best way to foil the Big Bad
's plan was to literally stop what they were doing because the villain's plans required the characters to do something
. Another plan in a different story was foiled because the heroine had made so many allies that the sheer numbers overwhelmed the villain's forces. Just because a villain has a plan doesn't mean he'll have the resources.