How should I play Cammy against a patient defensive Guile?

Divekicks are your main weapon here. Use them to get over Sonic Booms and gain ground, and then when you're in close use them to bait his Flash Kick or to get in if he's not blocking high. If he has to burn 2 meter to FADC something, that's a win for you. You can use these divekicks combined with crossups to force out Flash Kicks and then punish them.

After you get a knockdown, keep up the pressure. Learning how to add safe jumps, option selects, and unblockables to your mixup game will make you a very dangerous player; you can find guides and videos about this in plenty of places. Here is one example of a crossup divekick setup. When you crossup Guile, it's much harder and sometimes impossible for him to use Back-Charge, so after a knockdown you should be trying to do this.

Here are some VOD's of high level players playing the matchup: http://www.sf4tube.com/#guile/vs/cammy . The matchup isn't great for Cammy, but she's a very good character once you've learned how to vortex well.