Michael,
I'm green at spearing on scuba as well, in fact I've only done a little practice with my spearfishing gear in a pool and have yet to shoot fish, but where I live has allowed me to meet seasoned spearos and gather a ton of information. I highly recommend picking up the DVD, "Barebones of Spearfishing". There are some great tips and techniques in the instructional video and it's dedicated to spearfishing on scuba in Florida and more importantly, doing it safe. In fact, I recently met with Kevin at Ocean Rhino and he gave me a bunch of great tips and was extremely helpful.
The few tips I can offer from what I've learned (some already have been stated) are never attach your gun to your body. An 8 ft. thug taking your catch and wrapping you in the line and into a wreck or ledge obviously spells disaster. People have died from this exact situation. Instead, immediately after the shot put one arm though the bands and then retrieve the fish. Having a buoyant gun and shooting down on the fish is key as those two aspects basically get the line out of the way so you can retrieve the fish and spear. Shooting down into the bottom also keeps the fish on the spear rather than going through and it ending up on your line. Get a one handed stringer that allows you to put the fish on the tip before opening it. Ocean Rhino makes one. Bring a light. Don't freeshaft your first time. A scooter will more than likely spook fish away from the area (remember you are now a hunter). You wouldn't roll up on a deer with a four wheeler. Not to mention you would be heavily task loaded at 100+ feet. Gun, stringer, hopefully fish (that may require deflating as you accend), possibly sharks, your standard gear and then to top it off with a scooter. Also, the regs are constantly changing with early closures, etc. so stay up to date with what's legal. Don't poach.
Finally, I've learned and it's prevalent in tons of bad YouTube spearfishing videos, the most important thing in effective spearfishing is shot placement. Do some research on this and the anatomy of the fish and the effects of a spear. Stoning a fish makes the hunt so much easier and many times it's just a matter of a couple fin kicks toward the fish to get it to turn away from you on an "angle" which allows you to put a perfect shot on the fish above the gill plate and behind the eye in the soft tissue. This will most likely stone the fish or at a minimum pin it so it can't swim. I've seen a ton of terrible spearfishing videos of guys impatiently just shooting fish in the face, at a 90 degree angle, or ruining the meat by blowing up the side. Remember too that your effective range is only about two shaft lengths and our vision is distorted underwater looking through a mask.
Anyway, like I said, I'm new to this too, but I hope this help. Perhaps others can add to what I've said here.
Oh, one more thing. If the taxman shows up and displays interest in you, establish dominance. Swim toward him with your gun, not away. Most prey are always caught from behind running away.
