I hope you know how to adjust it properly at the start of the dive to get the perfect fit. If not, here it is:
When you gear up on a boat, don't worry so much about tightening things down. Just do it enough to get off the boat. For shore diving, pull your shoulder traps all the way so your tank is as high on your back as you can get it (makes it easier and more stable to walk into the water).
Once you're in the water, loosen all of your straps and begin your descent. At about 10', while horizontal in the water, pull down your Zena as far as it will go on your body. Starting at the bottom, tighten the straps snuggly, making sure that the middle straps are sitting at your natural waist below or right at the bottom of your rib cage (depending on where your waist is). It is very important than the middle stap is at your waist, and if you can't get it that low, you need a bigger shoulder section.
Once the body straps are tightened down (from the bottom up), snug down your shoulder straps. You'll find they don't need to be tight, just the slack taken out of them. If you've done this process properly, your Zena will NEVER move or shift on your body during a dive, regardless of what position you're in. It also won't ride up at the surface when your wing is inflated.
Also, as a side note consider getting an add-on lumbar pad for your Zena. It makes an already comfortable BC even more comfortable. There's also a bigger pocket you can buy that is specific to the Zena (and Zeus) if you need one.
I have 400 dives on my Zena and it looks as good today as the day I bought it. Even the color hasn't faded!!