Luckily I haven't had to do this and don't have to with my current certifications but what I would do is keep up-to-date with my knowledge throughout the years. If were talking about SANS specifically, and you took one of their courses, I'd recommend going back through the material to refresh your knowledge, compare the course outline from how it was then and how it differs now & just study up on the topics that you haven't learned. What's great about SANS is that their exams are open book (from what I have heard), this can help out a ton!
If you do have the funds to take the class again I'm sure that'd help out a ton but it's all about study, study, study!
+1. I just got into the cert arena a little bit ago and my re-cert is coming up for Cisco but they work differently. I write all my notes down in college notebooks, index them, and review them on a somewhat regular basis. I also throw in labbing on a regular basis (2-3 times a week) to keep sharp on older ways of doing x, y, and z and then try to implement newly released methods doing the exact same thing.