Thou shall not:
Spend More than you make. There may be times in your life when this was necessary. Few of us can buy a house or car without taking on debt and a total obligation well in excess of our cash flow. College education for the kids, major medical bills...life happens. Having the ability to borrow money and temporarily go into debt is OK. What can quickly ruin your retirement, however, is spending on wants and desires in excess of what your income is. The basic rules of finance don't get suspended once you cash your last paycheck. Funding your retirement with credit cards, home equity loans, or other options that put you in a perpetual hole will not end well.
Ignore the need for a budget. Closely related to the point above, I don't know how you can make it if you haven't kept and maintained a budget for years in advance of retirement. That need continues. In fact, when regular paychecks stop, tighter control over your income and expenses is even more vital. The old rule of thumb is you should plan on spending roughly 80% of what you spent before retirement. I suggest that "rule" no longer applies. You should develop a budget based on your resources and what you think you will spend. If those two numbers work for you, then the percentage is not terribly important. But, you must maintain a budget.
Assume others will take care of you. By others I mean the government, your old employer, your family, or winning the lottery. We have entered a new era where personal responsibility must be your primary care provider. It is likely you will receive some assistance in the form of Social Security and Medicare. If you have a pension you may receive everything you expect. Your family very well might be there for you every step of the way. But, I strongly urge you to plan as if none of that support will be there when you need it, not because I am overly cynical but because ultimately whatever happens will fundamentally affect your life, not theirs.
Make a retirement plan and never review or change it. An overworked cliché, maybe, but still true: the only constant in life is change. That is absolutely true when you retire. There is no way you can correctly anticipate what interest rates, the stock market, real estate, or inflation will do over the next 20 or 30 years. The political process guarantees unpredictability. At the very least, once a year take a look at every assumption, every budget category, and every projection of your future income. Adjust as required.
Become bored and restless. Too many retired folks go back to work because they don't know what to do with all the free time. Others spend their days in an arm chair, watching TV or reading 8 hours a day. Still others play two rounds of golf a day, not because they love it that much, but because it fills the time. This should not happen. Time is a priceless resource. Control over how you spend it is one of the biggest pluses of retirement. Find your passion. Find something to do that jump-starts you out of bed each morning. All too soon, you will wish you didn't squander something that can't be bought, can't be stored, and can't be replaced.
Treat a spouse or partner poorly. The entire dynamics of a relationship changes when one partner suddenly retires. To assume the person who just stopped working gets a free pass and can contribute nothing to the smooth operation of the household is not going to work. All that extra time together can be the greatest period of your relationship, or can contribute to the rapidly rising divorce rate among older Boomers. Work hard on your budget and finances...work even harder on strengthening your primary relationships.
Move right after retirement. The stress of retiring is substantial. Suddenly what your life looked like changes. Much of what gave you purpose and meaning is over. That is not the time to tear out the roots of your home life. Regardless of how much you want to move away from the rain or snow or desert, no matter how much you want to live near your grandkids....do not make that decision for at least a year. After the upheaval of not working settles down, then you are able to rationally look at what you'd be giving up and what you'd gain by moving.
Ignore your health. I don't need to belabor this point. If you don't feel well, if you don't take care of yourself, and if you don't follow common sense steps to maintain your health, your retirement will not be all you want it to be. If you already have health issues don't stop fighting for the life you want. You have a mind and you have creativity.You are alive and you are a unique being who have things to contribute and people to love.
Allows others to define what a satisfying retirement is (including me). This is a personal journey. Lots of people will tell you what to do or sell you a book with the 8 steps to a happy retirement. Heavens, I blog about the subject three times a week! But, the bottom line is retirement will become uniquely yours. Take all the input and suggestions you can. But, in the end, you decide what your life will look like.
Become a curmudgeon. The stereotypical grumpy old man (or woman) isn't a requirement of aging. Don't become so set in your ways that you reject everything new as flawed and no match to "the good old days." No one likes to be with a grouch.
Unlike the tablets Moses gave us, these ten commandments aren't written in stone. What have I missed that should be the 11th or 12th commandment? What in your view is the most important one on my list?