Ive been pushing the envelope hard for 15 years and never had an accident or a DCS hit. I could not have done this without a lot of training, as suggested, including several deep and tech diving courses. I also could not have done it if I didn't take the extreme diving very seriously and therefore work just as extremely to learn and understand the challenges, before trying it.
Just a few personal lessons:
1 - Be in the best shape you can - being lean and aerobically fit go a long way to staying safe, and conserving you air.
2 - Study hard and pay attention - focus on learning the material, not on how to get certified with the least effort.
3 - Thoroughly understand the science of gases - why each gas is used for what purpose, and how the different water pressures effect the gases - how the different mixes work is interesting and a bit challenging at first, but once you understand you can make a lot more safe judgements on how to dive. And by gases I also mean just plain air - the most important gas.
4 - Pay close attention to human dive physiology - understanding how the different gases get into and out of your tissues and organs is the key to diving safely in all conditions and depths.
5 - Be extremely serious about your training classes and training dives - getting it right while your with the instructor means it will come more naturally in the open water - and that brings incredible joy and ease to your scuba dive trips.
Regardless of your certification, understanding how your body is effected by breathing gas under lots of pressure, is key to diving. Then understanding what equipment and techniques will make your diving effort most effective, will make it easier.
The harder you work to learn the info and skills from getting certifications, the easier it is to put those skills in the water. The easier it is to put the skills to use, the better time you will have when you go to have fun. And the likelier it will be that you will get home safely (or back to the boat at least) - so you can do it all over again...
...enjoy and be happy... scuba rocks...