As a professional, I see a many stressful situations that could easily be avoided if you:
#1 Do a Buddy check: B - BCD, W - Weights, R - Releases, A - Air, F - Final OK. (or Total Diving System check).
#2 Check your gauge frequently on the dive: Once after 2 minutes and then at least every 5 minutes after.
#3 Tell your buddy or guide when you are at 70 BAR and again when at 50 BAR.
#4 Keep your skills current: Sign up for a refresher course if you haven't dived in 6 months or if you need a confidence boost.
#5 Dive within your limitations: Don't blindly follow someone to depths beyond your training or into overhead environments!
#6 Maintain positive buoyancy while at the surface and keep your equipment in place until your back on the boat.
#7 Plan your dives, ask questions about depth, dive time and no decompression limits!
#8 Invest in a high quality mask and snorkel: A very high proportion of stressful or panic situations is due to leaking mask.
#9 Prepare and organize ahead of time: Know the time before you arrive at your dive site or time your expected to be ready to be ready.
#10 HAVE FUN: It's meant to be fun, enjoy what your learning, relax, breathe and have a blast!
G. Head instructor, Scuba J