I think you need to consider both intrinsic and extrinsic motivators as ways to improve your ethics. One or the other may work best for you, or even a mix of both.
For me the intrinsic motivators are most effective, though maybe this is because I'm an introvert with a rich inner world :) YMMV. In any case, an intrinsic motivation for ethics is focused on how your actions affect others and your internal view of self (as opposed to external consequences/rewards). In essence this is empathy. How would you feel about someone who's always trying to take advantage of you, trying to scheme against you? Is that how you want to treat others, including employers?
In the case of someone or some employer mistreating you (e.g. feeling underpaid, someone cheating you) the ethical question is still "is this how I think people should be treated in general?" And so returning the favor, so to speak, is lowering your internal ethical standard to to their level which then erodes your own integrity. The ethical response is to treat them with the integrity they do not deserve while seeking proper recourse (finding a new job, legal options, so on).
If, however, you have Antisocial Personality Disorder (lacking empathy, concern for others, etc.) then seek therapy.
As for extrinsic motivators, you need to consider how useful your scheming actually is relative to other possible efforts. I had a friend in highschool (damn, that was a long time ago) who was always scheming, usually very gray area ethically and legally, occasionally outright illegal. I was always amazed at how much effort went into these schemes as compared to how little they yielded. In the end he would have been better off instead focusing on getting a degree or learning a career skill.
Similarly, I have a relative in law enforcement who often comments at how hard criminals work at being criminals. Almost all would be better off just getting a minimum wage job. Would be less work and make more money, all while staying out of trouble.
In the case of your gift card scheme, how much time and effort did you have to put into the $1500 payout, and how much income did you lose after getting fired while finding another job? In other words, is your behavior actually beneficial compared to the more ethical life? <-- I think this can be effective, though I find it unsatisfying because there are instances where unethical behavior produces a much higher monetary payout even though this erodes one's self. So for this reason I still prefer intrinsic motivators.