The road down there from Aswan is considered high risk, and currently inaccessible to foreigners travelling in their own car. However, you can travel there in police-escorted bus convoys, OR rent a car with a driver from a local travel agency in Aswan, who is familiar with the roads and rules.
There's at least one daily convoy a day, and takes three hours to get there. So once you've got your transport sorted, you'll want to choose one. If possible, you want to get there before sunrise. This isn't always possible depending on the time of year, but if you can, as soon as the bus stops, RUN, and don't stop running until you get to the site. Take your outdoor photos of the sun rise hitting the temple FIRST before the hordes of other tourists get there, and THEN go inside to explore :)
Apparently it's also possible to go by cruise ship through the lake, but I have no more information on that :/
It's a beautiful site, and my highlight of Egypt (yes the pyramids are great, but this site is magnificent). Don't skip it! :)