That stuff can be hard on the back and neck. My fave book is by the Mehtas on some Iyengar poses and they are broken down into sets for various ends, with good pics and then you really take it slowly, and do some weight sets in there and maybe walking or some other heart-stamina thing. Then take bathes and soak out ones muscles. For hatha yoga one likes to be relaxed as possible.
I like sun salutes (surya namaskar) for energizing. Even a few half sun salutes helps tremendously when I'm fatigued. I don't care at all for the 5 Tibetan rites.
Cool thing with surya namaskars is doing them with the 12 sun mantras which increases awareness of the sun vibration and also works breath into the exercises. http://www.iloveindia.com/spirituality/mantras/surya-namaskar-mantra.html
A good doctor diagnoses before he writes a script. How did you come to lose your energy level? What is your day like? x
As far as a simple exercise, and I'm assuming you'd like to increase your energy levels, you might like to try Prana Mudra, instructions in which can be found here: http://www.flickr.com/photos/21678462@N00/3018539920/sizes/l/ http://www.flickr.com/photos/21678462@N00/3018540020/sizes/l/ They are taken from the text "Asana, Pranayama, Mudra, Bandha" May I also suggest "Cultivating Prana" by Robert E. Svoboda