Unless you're an investor who enjoys taking higher risks for higher rewards (that may be the case), a great way is to simply start buying as the index hits bear territory and continue buying until you reach either a sizable position or the sector recovers.
There are some sector specific ETFs as well as broader ones covering multiple energy sectors (drillers, pipelines, refiners, etc). It doesn't take a genius to realize that energy is weaker than it has been in a long time, and chances are good that it will go higher rather than lower in the next few years. Of course, no one knows for sure, but that's around as certain as it gets with commodities.
Personally, I'm researching into the drilling companies as well as equipment/services, since they've been hit among the hardest. Additionally, the integrated majors have come down a bit, but much more slowly than other sectors. You don't have to always buy at the absolute bottom in order to enhance your returns in a reasonable manner.