There are some tricks to the Zena, it's good you asked. Making sure it is adjusted properly on your body will guarantee that your tank will never rotate or move on your body during the dive. Before you buy, make sure you're positioning the straps properly on your body wearing whatever thickness of suit you normally wear. Zeagles are modular in design so you can mix and match the 3 pieces (vest, front panels, and shoulder section) for a near custom fit. Keep in mind that if you cannot get the middle strap to your waist, you need a larger shoulder section, and if you can tighten your shoulder straps ALL the way down while the middle strap is positioned at your waist, you need a smaller shoulder section.
Here's the quick tutorial:
When boat diving: Make sure all of your straps are loose before zipping up the corset. Once zipped, tighten everything down enough to get off the boat. At the start of your descent, go horizontal and loosen all of your straps completely. Grab the sides and yard the entire BC down until the middle straps are below (or at the very bottom of) your ribs or at your natural waist. From the bottom up, start tightening the straps, again making sure the middle strap is at your waist. You should find that you simply need to take the slack out of your shoulder straps. If you've done this correctly, you should have about 1/2 of the slack taken up in your straps and there should be no movement of your Zena or tank no matter what position you are in.
When shore diving: I've found that if you tighten the shoulder straps ALL the way down immediately after putting your tank on your back, walking to the water is much easier. I also tighten them down before exiting the water as well. The process used to adjust the Zena is the same once you're in the water.
I did my rescue class in my Zena and had no problem removing or redonning my BC during the exercises. I simply had to remember to loosen the body straps prior to removal and before I re-donned it. I have over 500 dives on my Zena and it shows little to no wear, and I'm looking forward to another 500 dives in it!
If your LDS won't custom build a Zena for you (at no additional cost), consider taking your measurements and calling the guys at Scubatoys. Not only will they build you a perfectly fitting Zena, but they will guarantee that it will fit or they'll make it right!