Raising stakes is important. Janice Hardy talks about it better than I ever could, but I did have some recent thoughts on the subject that I thought I'd share.
One of the problems with trying to raise stakes according to Donald Maass, author of Writing the Breakout Novel, is that it's too easy to have the same stakes only bigger- like, first New York is in peril, then later all of North America, then as the climax nears the heroes discover that the World is Doomed. Of course, the reader has an easier time picturing an imperiled New York than the world doomed, so by the time the high stakes are reached his or her brain just exploded.
There's another problem with raising stakes that way though; the emotional play is too similar. "Oh no! Not New York!" "Oh no! Not North America!" "Oh no! Not the Entire World!" The repetition can make everything flatten. But sometimes the story really does want to go, "First New York... then North America... finally... the Entire World!" Donald Maass suggests that one way to fix the problem is to have new complications mixed in -like family problems, or rioting. That is very good advice.
Another thing that can help, though, is varying the reaction of the characters. I have been working (or pretending to work) on a story where there is a monster attack. A couple chapters later there is an attack by an even bigger monster, only when I got to that scene the second monster felt anticlimactic. There are, after all, only so many ways you can say 'my hero was terrified. He was really terrified. Seriously folks, this is scary stuff.'
Fortunately, God blessed me with friends who aren't afraid to tell me when my story is not working so I went back to the scene, stared at it a lot, banged my head on the desk a few times and finally thought what if the hero isn't terrified? What if, instead, seeing the monster gives him an adrenalin rush that makes him unusually reckless? It's a reaction that makes sense, it builds suspense (because reckless characters might make mistakes) and best of all it's not just a repetition of the earlier scene. I don't know that varying the reaction will always work to raise stakes, but I know it worked for me in this particular instance and I'm filing it in the back of my mind for future use.