I'll tell you why things didn't end badly for me yesterday in highsec: It's because they ended badly for me the day before that.
I knew I couldn't just ambush ships in highsec like I do in lowsec. Concord takes it's job very seriously, and attacking a ship for no good reason in highsec is a way of asking Concord to come against your T1 frigate with a flotilla of frigates, destroyers, cruisers, and battleships. It's suicide, certain death. But "no good reason" implies there are good reasons, and that's the trick.
If you're mining an asteroid, and some pirate comes along and steals ore out of your jettisoned cargo container, you have Concord's permission to initiate violence against that pirate. But (and this is important): if you're that pirate, and some miner attacks you, you have Concord's permission to fight back! In either case, you have to act fast--Concord only recognizes such kill rights for 15 minutes.
If you're lawfully engaged in combat, and destroy an NPC pirate ship, Concord recognizes your rights to the spoils; other pilots are free to salvage the ship itself, but if they loot whatever modules or cargo that survive, you again have kill rights for 15 minutes.
So, highsec piracy is a matter of getting kill rights on someone you can beat. We pirates want people to steal our ore, open fire on us, or loot our wrecks!
I first gained experience in this just the day before yesterday's successes. I worked a series of asteroid belts, leaving unlooted wrecks in my wake; but after a couple of hours, none of the miners had fallen into my trap. So I fell back on a classic highsec pirate tactic; I found a couple of miners who were storing their ore temporarily in jettisoned cargo containers (known as "jetcans" in the trade), and I simply jettisoned a can of my own nearby and transferred their ore from their can into mine. Technically, they had the right to kill me for the next 15 minutes; in practice, however, most miners think twice before attacking a combat-fitted frigate. After all, their ships are probably fitted more with mining lasers and less with guns and scout drones.
I then left the system, giving the miners time to feel safe and opportunity to get their ore back. If any did take their ore back, the interesting thing is that Concord would, from that moment, give me the right to kill them for the next 15 minutes. After about 15 minutes, I re-entered the system. According to my scanner, some of the miners were still there! I warped in on top of them, and they were flashing red! They'd reclaimed their ore--and I had the right to do something about it. I quickly locked my target and in a few short moments I had prevailed.
I had to deal with a couple of NPC pirates before returning my attention to the wreck of the mining ship I'd destroyed. These ships can have millions of ISK worth of modules! As I approached the ship, though, the pilot warped in on top of me; only this time, instead of piloting a relatively defenseless mining barge, he was in a frigate armed for combat, fitted with missile launchers. I was glad of the research I'd done before selecting my ship and its fittings, however, and the skills I'd gained in operating it efficiently, for once again I prevailed, even though I'd just finished three other fights. Thinking "I'll teach him a lesson about coming back here," I targeted my victim's pod this time, and had no trouble destroying it also. Then, inexplicably, as I turned yet again to loot the juicy wrecks showing on my overview, my shield and armor indicators went red, and the next thing I knew I was dead!
I had no idea what had happened. I didn't see anyone else in our vicinity. Had my victim launched some super (but slow) missiles which only then reached my ship? It was hard to believe. Only when I got a kill mail from Concord did I learn another important lesson about highsec piracy: It is never acceptable to destroy anyone's pod; by attacking my victim in his pod, I'd brought the wrath of the authorities down on my head!
I am so glad to be learning these lessons while I'm piloting frigates, rather than later when I may be in a battleship. (Or so I tell myself, hoping to dull the pain of yet another loss.)