The easiest way I know of to shift weight lower without resorting to ankle weights is to use a Marseille-style rubber weightbelt favored by freedivers. The Marseille buckle is nothing more than an overgrown buckle like most men use on clothing.
MAKO Freedive Weight Belt
The advantage is you can stretch the rubber belt much tighter than with a conventional cam-buckle used on typical Scuba weightbelts. That allows you to wear the belt far lower without slipping off. No joke, they stay wherever you put them as if they are stapled to you — even on a no-butt geezer like me.
Edit: Definitely look at this video:
http://www.youtube.com/watch?v=XwA8DL6-Ya0&feature=player_embedded
You can’t always depend on getting heavier steel tanks on charter boats or when traveling. You can compensate for aluminum tanks by bringing an extra cam band with one or two weight pockets that can clamp between the bottom and your BCD band(s).
Fore and aft trim is the biggie, but don’t ignore rotational trim. My ideal trim is between horizontal and about 15° up and to be face-down, on my sides, or face up in a mid-water dead-man pose with no air in the wing. That is where the weight of a stainless backplate can be most useful.
In general, I recommend limiting the amount of weight on a belt to no more than 10 Kg/22 Lbs. I arrived at that number by dropping progressively heavier belts while wearing a 2-piece ¼" wetsuit (in the old days when the rubber was US made and didn’t compress as much). That was my limit to feeling comfortable controlling ascent. Your mileage may vary. I also find that is about the limit for rubber belts so the weight itself doesn’t stretch it too much.
Ideally, I suggest getting in a swimming pool where you can help her shift weight until you find the sweet spot for her gear configuration. Move the belt up and down, move weight around on the tank (an Aluminum 80 is probably the worst case), and even play with ankle weights as a last resort (you can probably borrow or rent them). Weight pockets are great for tests like this even if you don’t use them later.
An experienced eye of an instructor or another diver “might” be useful, but I feel that both of you will learn a lot more by doing this yourselves. It is a simple matter of balancing displacement and weight. Talking to new divers, I sense the concept is intellectually understood even if they never heard of Archimedes' principle. However it is not intuitively understood. Until it is you will never be able to adjust your ideal trim with suit, fin, and cylinder changes. It will be time well spent.
