One of the first things you hear when you begin playing Eve, the "Prime Directive" so to speak, is "Don't fly what you can't afford to lose." Spend any time reading the forums, reading Eve blogs or visiting trade hubs and you are guaranteed to hear this repeated at least 3 times a day. And with Hulkageddon right around the corner, expect that to increase by a factor of 4.
When I started out in the game, a wide-eyed capsuleer staring at space upon tantalizing space, this was pretty easy to follow and understand. After all, rookie ships were free and t1 frigs were only marginally more expensive. But as I matured as a pilot, it was a consideration I had to take on with increasing frequency. Over time, it wasn't Navitas' and Imicus' I was risking but Vexors, Myrmidons and later, MUCH later, Ishtars and Proteus'. Could I really afford to risk them? These hulls which cost me more than all of the frigs and cruisers I had purchased to that point combined? The answer, ultimately, was yes, but I arrived at that conclusion in an unexpected way.
In the early days of my Eve career, back when I had maybe one or two ships in my hangar at any one time, losing a ship meant replacing it and replacing it quickly. Like, within minutes of waking up in a vat of clone goo. But as I progressed and built my wallet, the loss of a ship affected me only to the point at which it impacted my ability to make ISK. It didn't matter if I lost a ship so long as I had other ways, that is to say other ships, with which to keep the coffers full and the revenue stream flowing.
The other interesting aspect to this is that since those early days of rookie-dom, I have a vastly deeper understanding of HOW to make ISK, and make it quickly. I know now, in a way I never could have as a green, where my limits are. I understand the risks inherent in my various activities, I understand replacement costs, and I understand the bare minimum of what I need in terms of ISK, ships, and mods to recoup a loss in a reasonable amount of time.
And that's really what "Don't fly what you can't afford to lose" is all about. Understanding, preparing for, and ultimately mitigating the risks that can and will materialize as you journey through space.
Now don't get me wrong. To this day when I lose a ship, I usually replace it within a week. Not because I have to, but because the ship layout and fit was typically one that I tailored to do a job better than any other ship I have skills for could. The delay is merely convenience factor. But back when I was living in null, and roaming reds were a daily, sometimes hourly occurrence, I lost at least half a dozen Myrms and who knows how many 'ceptors and bombers. And I replaced them often. But living in null afforded me the ability to make ISK at a pace that permitted me plenty of leeway on replacement schedules and with a wide variety of ships. I undocked and risked these ships not because I could afford to replace them, but because I could afford to do without them period. That rule I won't ever break.
I'll have to admit, it's a much easier distinction to make when talking about Myrmidons than it is when a shiny faction-fit T3 is on the line. But the principle remains the same. When that next mission or plex is looking risky or the next lowsec gate smells a little camp-y, don't fly the ships you need to live. Fly the ships you can afford to live without.