If you own your own business, the best answer is to turn this business into a source of passive income. The way to do this is to turn over the daily operations of your business to your trusted employees, and then to slowly even turn over the leadership and visionary aspects of it to someone whom you have developed over time. And you can do this while still retaining substantial ownership rights and receiving a monthly dividend from the profits of the enterprise. If you are a current business owner, plan for this form of exit so as to secure strong monthly returns on your investment through your financial freedom years.
For most others, the need for monthly returns arises once portfolio money from retirement accounts becomes available for investment. There are several strategies one could employ, and perhaps a good mix is in order. The simplest answer is to either put the money to work in Certificates of Deposit, or in Municipal Bond funds. The latter money is not taxed at the Federal and sometimes State level, and thus boosts your returns (but rules vary). Bond funds, including municipal bond funds, give out interests on a quarterly basis, though some may pay on a monthly basis.
Moving up the risk ladder, you could seek out income oriented funds - and here you have many choices. You could go with funds that invest exclusively in bonds (long term, intermediate term, corporate bonds) or those that are more oriented towards real estate (REITs), or those that are oriented towards dividend paying stocks.
Furthermore, you can choose between closed-end and open-ended mutual funds. There are several closed-end funds that invest in a variety of strategies at the same time (debt, equity, options) in an attempt to boost their payout to you. Some carry more risk than others, but with some good guidance (from advisors or newsletters) you should be able to easily preserve your capital while getting above-average cash market returns.
Such a portfolio should ensure an above average returns (higher than CDs or municipal bond funds), and reduce risk considerably. Always do your homework before making any investment, and do not hesitate to seek out financial advisor or investment newsletter help in order to improve your monthly returns.