The only rules I will ever follow:
I will not masquerade as a hero. My Twinkling Eyes and Rakish Grin would give me away all too soon.
I will not masquerade as a villain. My Heart of Gold would surely betray me at an inopportune moment.
Swashbuckling with swords is good exercise and good for publicity. Even so, if I have the opportunity to dispatch a foe with a throwing knife or a crossbow bolt from the shadows, I will take it.
I will not make a move on the hero's One True Love. Either the hero would come to her rescue and give me a sound thumping, or, worse, she would beat me down herself.
I will not make a move on the hero. As much fun as it would be to watch him sputter and blush in front of his companions, the scene could only hurt my chances with the ladies.
I will bring an extinguisher and a crowbar to every heist, so as to claim even those things which are nailed down and on fire.
I will hire a man to equip, activate, taste, and randomly fiddle with any extremely unusual items I acquire. If he's still alive after a couple of hours, I will take the items and pay him for his services. If not, I'll bury the items and console myself with his wallet.
I will occasionally donate some of my loot to charitable causes, so that the hero (if I am ever at his mercy) will have qualms about killing me.
I will avoid the hero on weekends and social occasions. No matter how well I've earned my reputation as a ladies' man, when the good guys are nearby, even the naïvest of tavern wenches will inexplicably be immune to my charms.
Accordingly, I will remember that any woman who shows an interest in me when the hero is around almost certainly has an ulterior motive. I'll still take her out on a date, of course, but I will refuse any seemingly innocuous tasks, requests for information, or beverages she offers me.
I will not steal from tombs, crypts, altars, or any person who manages to look menacing while dead.
Furthermore, no matter how shiny the treasure is, if a booming disembodied voice tells me to put it back where I found it, I will do so promptly.
Before attempting to pick someone's pocket, I will first tie his bootlaces together, thus reducing the danger to myself if my legerdemain is detected.
I will not keep a journal of my exploits. A journal's sole purpose is to be found on its owner's corpse in a dungeon somewhere, and I'd rather not have adventurers tsk-tsking over my unorthodox lifestyle as they loot my hard-stolen belongings.
I will take credit for all of the hero's exploits in song.
When haste is my top priority (for example, when hundreds of tons of stone are crashing down around my ears as I scramble for the temple exit), I will remember that haste is, in fact, my top priority, and I won't try to drag a big bag of loot along with me. If the treasure is that great, I'll hire an excavation team to dig it out for me later.
I will realize that a cute animal sidekick is just as likely to get me into trouble as it is to help me out of it. Thus, before accepting one as a companion, I will at least make sure that it's smart enough to make itself scarce when I'm talking to the ladies. Nothing kills romance faster than a rat up the skirt or a monkey on the head.
I will not seduce the Evil Empress in an effort to obtain classified information from her. Whether she sees through my plan or not, she'll most likely use her mind-control powers to turn me into an unquestioning slave. Though my duties would surely be pleasant, a rogue is nothing without his freedom.
I will not act as a double agent for either side, since such activities are invariably discovered. If the Evil Overlord caught me double-crossing him, pain and death would be imminent; whereas if the hero learned of my betrayal, I would have to apologize and make some sappy, extravagant gesture of loyalty to redeem myself. Both scenarios are unacceptable.
I will familiarize myself with the local laws and ordinances of a town before I steal anything from its citizens. If it turns out that nicking an apple from the market gives the shopkeeper legal clearance to cut off my hands, I'll pass.
If I ever start dating a good and beautiful princess, I will take the first available opportunity to let her know that I'm not really a prince. Once she has fallen in love with me, she won't mind that I'm a common rogue, as long as I'm the one who breaks the news to her. Whatever happens, I won't let some rival suitor find out before she does and then blackmail me about it.
I will not alternately mock and flirt with the strong yet attractive bounty hunter who is chasing me. It never helps. (And if I'm caught, I'd rather be taken alive, thank you very much.)
I will not let my tongue get sharper than my sword. Nothing hurts worse than losing to a guy right after smack-talking him up one side and down the other.
No matter how dashing I look in the traditional garb, I won't wear pants so tight that they impede mobility or shirts so poofy and unbuttoned that they get caught on things.
(stolen without permission from True Magic the online comic)