At present, I own two properties with eight units. Bought about 15 years ago.
What I did right: Bought properties with fantastic cash flows right from the get-go. One property, we paid $55K cash and it flows $1100/month. The other, we put 3% down on a $130K five-plex property, lived "free" for a year in the sense that our tenants paid the mortgage, which allowed us to buy a cheap SFR.
Another thing we did right (for us) - Refinanced the $130K property into a 15-year mortgage, which still provides us with good cash flow and will be paid off in two years, just in time for state college tuition for our oldest child.
What I wish I'd done differently - We sold the SFR when it would have been a good cash-glowing rental. I wish I'd kept that and bought 1-2 more SFRs before we stopped buying, because we'd be nearing having them cover all our expenses rather than about half our expenses.
I wish we'd hired a property manager a lot sooner than we did. We likely would have bought more property if we hadn't had to put time into the rentals.
I won't go so far as to say that buying income properties is a young person's sport, but if you do it right when you're young, the incentive to buy more when you're older (I'm mid-40s) is way less, at least for me, which is why I wish I'd bought more when I was in my 20s and early 30s.
Another thing we did that worked for us is we never took equity out because our goal was to have our properties paid off in time for our kids' college educations. (Hopefully, they'll get scholarships and we can party with the money instead...!)
I find I'm less willing to take a leap and buy another income property right now because at this point, all we have to do is NOT make any mistakes and we're set in terms of our financial goals. So I guess to sum it up - we got great deals when we were young, we held onto good properties (with one exception), and we had a reason for financing the way we did and not taking out equity.