As we are all well aware Google makes consistent algorithm updates throughout the year and some of these can be only little updates having very little effect, others can be HUGE, major even and can fluctuate your rankings quite substantially.
However if you are doing SEO the right way, your rankings shouldn't fluctuate too much. Creating credible quality content on your own website which is naturally linkable is the present and future of SEO. If you are continuing to buy links in large quantities which are irrelevant then Google is going to come after you!
The big questions is though, and which is answered very well by Ryan Shelley on Search Engine Land, "What do I do if I see my rankings fluctuate after an algorithm update?"
Well take it from us and Ryan, relax, wait, be patient and if your rankings don't return after a while then look to credible resources for information on what changes you should and shouldn't make.
After relaxing and not panicking, the next thing to do is wait to react. It can be tempting to go out and make changes to your site or process, but that could be wasted work.