There are a lot of people doing deep dives with relative safety. The OP asks how to prepare himself to do them.
First off, you have to understand what changes when you go deeper. Water is water; it feels the same and often looks the same, so what's different? You were taught in your OW that you use your gas faster at depth, but the changes are pretty remarkable. The tank that lasts an hour at the surface, lasts a half hour at 30 feet, 20 minutes at 60, and 15 minutes at 100, and 12 at 130. That's 12 minutes to EMPTY . . . it doesn't include an allowance for descent or ascent, keeping some gas in the tank, or having any reserve for a delay or an out of gas buddy. I would highly suggest studying some gas management (you can begin with the gas management article at nwgratefuldiver.com) before contemplating deep dives like this.
Second, nitrogen absorption begins to become a big issue at such depths. It's unlikely the typical recreational diver is going to go into deco on a 60 foot dive -- the small tanks and high gas consumption rates in such settings pretty much preclude it. But your no-deco time at 140 is very brief, and maybe shorter than your gas allowance. Once you have incurred a mandatory decompression obligation, you can no longer go to the surface in the event of a problem. Therefore, you should be comfortable managing all the typical malfunctions -- flooded masks, wet breathing regulators, freeflows, disconnected inflators, loose connections, etc. -- at depth, with composure and preserved buoyancy control and situational awareness, before you trap yourself underwater. For most people, it takes a fair amount of diving and some stress-scenario training before they are really competent with this.
I personally do not understand the drive for depth, as I don't have it. The majority of ocean life lies in the shallows, and diving there is less hazardous, as well as allowing much longer dive times (and why do we pay the money and haul ourselves to dive sites, if not to revel in being underwater?). But we all dive for different reasons, and if depth is your draw, prepare yourself properly for it. To me, a 140 foot dive should not be done on a single Al80 (go back to Bob's gas management piece), but is a totally acceptable dive if done with redundancy, an adequate gas reserve, a contingency plan for decompression, and solid diving skills.
