Never sell a service or product that you cannot deliver. Never sell a service or product that the buyer doesn't absolutely need or love. Be essential or desired, not annoying and unnecessary.
Never work for someone who isn't as smart as you are. Or plan your exit the moment you figure out that you have learned all you can and that you are now the smarter one.
Never work for or with people with a lesser moral code than your own. Be aware of how your colleagues feel about doing the right thing. You should watch how they prioritize it. Once you determine that the moral failings of the people around you are systemic and indefatigable, it's time to get going. If you can fix a bad situation, by all means try. If you can't, reserve judgment and simply say goodbye.
Never work in a career that relies on opacity, obfuscation, rhetorical fallacy or sleight of hand. There are plenty of people who can do this sort of work, fooling their neighbors and customers or tricking them into transactions that aren't what's in their best interests - the key is to not be one of them. Those who engage in this sort of work are either sociopathic or trapped because of financial circumstances or too stupid to have thought the consequences of their career choice all the way through.
Never cut any corners, there is no such thing as a free lunch and everything has a cost, even if you can't see it right in front of you. Riskless reward is a desert mirage.
Never pursue something that you really don't want in the first place just because you think you have to. You don't have to and it won't work out anyway. Successful people become successful because they are doing what they love and have a talent for.
Never keep a bad client just because they're willing to keep paying you. Never allow a mismatched customer relationship to skew the way you do business or take care of your other clients. Never put off firing a customer the moment you realize there is a bad fit and that neither of you will be satisfied in the relationship. Life is too short to do business with unreasonable people or nice folks whom you just cannot make happy.
Never go through the motions. Find a psychologically rewarding way to go about your tasks, remind yourself constantly where the day-to-day drudgery of your job is leading. If it's leading nowhere or toward something you don't truly want, stop immediately. Don't spend a moment being busy for no good reason.
Never taunt others when things are going your way, people like dealing with gracious winners who raise others up with positivity. Never burden others with your problems when things are not going your way, the amount of mileage you'll get out of pity is minimal and people will go out of their way to avoid getting involved with you.
Never watch the clock or calendar. Have reasonable expectations for the timeline of your success. If you enjoy what you're doing and are going to work with purpose each day, then what's the rush? Only people who are doing something they hate are worried about how many dollars they can rip out of the endeavor right away.
Never believe for one moment that your path is already laid out for you or that you can't break away and find your own road toward happiness and success. Remember - Fate is the cards you're dealt, Destiny is how you play them.