Countless Americans look forward to retirement and the opportunity to leave their careers behind. But if you're not careful, a few critical mistakes could ruin your chances of enjoying the comfortable retirement you've been dreaming of. To avoid falling victim, study these three retirement rules -- and follow them closely. 1. You'll need more than Social Security One major retirement-related misconception is that Social Security will suffice in paying the bills for the typical senior. But this actually couldn't be further from the truth. In a best-case scenario, Social Security will replace roughly 40% of the average worker's preretirement income. But even if you're willing to live a relatively frugal lifestyle as a senior, that amount won't come close to covering your bills. The average beneficiary today, in fact, collects just $16,320 a year, but since the average healthy senior will spend $200,000 (or more) on healthcare in retirement, that $1,360 a month won't go very far. IMAGE SOURCE: GETTY IMAGES. That's why you'll need to save independently during your working years if you want a shot at a comfortable retirement. You don't necessarily need to set aside huge amounts each month (though the more, the better); you just need to save consistently and invest that money so that it generates a decent return. The following table shows how much you might accumulate over a 30-year period (which, if you're like most people, isn't even your entire career) based on your savings rate: Monthly Savings Amount Total Savings After 30 Years (Assumes a 7% Average Annual Return) $200 $227,000 $300 $340,000 $400 $453,000 $500 $567,000 $600 $680,000 TABLE AND CALCULATIONS BY AUTHOR. Note that the above calculations assume an average yearly 7% return on investment, which is more than reasonable with a stock-heavy portfolio. And with a 30-year savings window to work with, you have plenty of time to ride out the market's ups and downs and come out ahead. 2. You can't count on spending less Another common retirement myth is the idea of your costs dropping dramatically once you stop working. After all, you'll no longer need to pay for commuting, nor will you have to contribute to your 401(k). But what many people fail to realize is that living costs have a way of creeping up in retirement -- so much so that nearly half of senior households wind up spending more money, not less, during their first two years of retirement. Not only that, but for 33% of seniors, that trend continues six years into retirement. Part of it boils down to rising medical costs. Those who previously had comprehensive, employer-subsidized plans often see their healthcare expenses go up once Medicare takes over. But another factor at play is the amount of downtime retirees suddenly find themselves with. It stands to reason that when you're not working eight hours or more per day, you'll need more ways to occupy your time -- and that entertainment can cost you a bundle. Now this isn't to say that you won't manage to afford those extra expenses if you save appropriately. Just don't forget to budget for them along the way, and make sure that your savings level is high enough to buy you the lifestyle you want before you leave the workforce permanently. 3. You need a plan The Institute of Economic Affairs reports that retirement increases the likelihood of suffering from clinical depression by 40%. That's why it's crucial to not just plan for the financial aspects of retirement but for the emotional as well. Before you pull the trigger on retirement, make sure you have a plan for how you'll spend your days. Will you travel? Open a business? Or perhaps play the role of caregiver to your grandkids so your adult children can more easily go back to work? Your options are virtually limitless, but the key is to not go in blindly. This way, you're more likely to wind up satisfied with your new lifestyle rather than grow to resent it. Retirement is a milestone so many of us work hard for. Follow these rules, and with any luck, it'll bring you the joy and fulfillment you deserve. The $16,122 Social Security bonus most retirees completely overlook If you're like most Americans, you're a few years (or more) behind on your retirement savings. But a handful of little-known "Social Security secrets" could help ensure a boost in your retirement income. For example: one easy trick could pay you as much as $16,122 more... each year! Once you learn how to maximize your Social Security benefits, we think you could retire confidently with the peace of mind we're all after. Simply click here to discover how to learn more about these strategies.The Motley Fool has a disclosure policy.