Be conservative on your expectations - Always overestimate the amount it will cost to renovate a property, underestimate the rental income you will earn, and overestimate the expenses you will have to pay. Then, when you blow it out of the water, you'll have a big smile on your face and be able to gloat to all your friends!
Stay away from high vacancy areas or declining cities - Buy properties in nice, moderate, working class areas where there are not too many foreclosures, empty properties, or in a city or area that is in decline.
Start young, but not before you are settled in a particular city - You want to start early, but make sure you're somewhat settled before you take on this big responsibility. Have fun when you are young, there is still plenty of time to get rich on real estate. Just start saving your pennies for that first down payment.
Death, taxes, and..... - Those two items are guaranteed in life. But, there is one more item if you are a real estate investor, and it's 100% guaranteed: If you own real estate, you will feel pain along they way. Things will go wrong, but you will recover and will likely look back on the journey with fondness when you retire early.
And let's end where we started, once again with the most important item in real estate investing:
Go long! - Long-term ownership will give you the highest chances of entering retirement with a nice rental property cash flow stream. And long-term ownership equity gains will compensate for the hassles and issues you have along the way.