There are several things that I can think of off hand that may be worth trying.
- If you are not wearing a weight belt then put one on. Even if you don't put any weight on it then just having the belt around your middle will do two things for you. (a) it will create a restriction which will slow down the migration of the bubble in the suit and (b) it will give you a way to pull excess material, if any, from the lower part of the suit up above the belt creating relatively more room for the bubble above the belt. In other words, you can use this to some extent to play with where the air goes in the suit.
- you need to do a weight check. If you have too much weight then you will have too much air in the suit that will lead to all kinds of complications.
- you can consider splitting up where you put the air. It sounds like you might be using only the suit for buoyancy but you will get a very different result if you just put enough air in the suit to alleviate the squeeze and put the rest of the air you need in your BCD. You can play with this.
- you can move weight down lower on your body. You already thought to move the tank down lower but there is more you can do. You can attach a weight to the lower part of the tank using a separate cam-band (do not use a weight belt for this or it will fall off). Putting weight on a weight belt will put it lower than if it were in weight pockets. Ankle weights are an option until you get used to diving dry but personally I would tend to view them as a temporary learning measure (like training wheels). Gaiters that keep air out of the lower part of the legs by creating a constriction are another such tool. Heavier fins will work to some extent as well, particularly if you have plastic fins now.
- your body position has something to do with it as well. Pushing your belly out (holing your back) will cause your hips to rotate and in the process it will drag your legs down. Most drysuit divers use this quite a lot in order to fine tune their balance as they go. I use it especially when my legs are above my head and I want them back under me again.
- allowing your knees to bend so your feet are up is also helpful. Don't fight what the suit wants to do so much as find a way to manage and balance that.
- Where your head goes, the rest of your body will probably follow. If your feet are getting too far up and you feel like you're losing it then you can look up, hole your back as I described before and usually that brings your legs back down again. In very rare cases I've seen people somersault out of it and we do teach this technique but it's rare to ever need it once you have the knack of it.
- In terms of venting the suit. For now, try going mostly vertical to vent it. You also need to be patient because the suit will vent more slowly than a BCD will. This is also another argument in favour of putting at least some air in the BCD in order to make quick corrections if you didn't anticipate a buoyancy fluctuation quickly enough.
On the whole where you place the weights will have the biggest impact for now, so try all of that other stuff but focus in the next dives on trying to fine tune where you attach the weights to your set. And don't forget to do a good buoyancy check to make sure you're not making your problems worse by having too much weight.
R..
