Avoid all out of state investing. I went to 5 states and they all turned out to be the worst performers in my portfolio. The lost opportunity cost of not using that money to invest locally is incalculable.
Never partner with someone with less experience and money. What a complete waste of time.
Sell off my worst performing properties immediately. Never wait and expect it to get better. It doesn't. Drag them out behind the woodshed and beat them mercilessly into the ground with a big wooden club.
If it doesn't feel right, just don't do it. Your gut is much smarter than your brain. You'll be surprised. Live by, "It isn't the ones I don't buy that hurt me."
Avoid niche properties like condos and senior anything (houses, mobiles, condos). In a good market I will do them, and am doing one now, but in any other market these types of properties just take a ruthless beating and are next to impossible to sell.
Focus on cash flow needs today and get that covered ASAP. I still have a Post-It Note stuck above my desk from my early years that reads, "3 IDEAS to increase income $20 and decrease expenses $20." Do this once a month, especially when looking at the credit card bill. Those $40 gaps in the beginning make a big difference over time. I had another one, long gone now, that said, "$100 net income per month." Every month my goal was to increase my net by at least $100. If I did $150, I did not get $50 credit for next month. If I missed my goal that month, I owed it the next month. Once you meet your monthly nut, every other dollar is an extra point you don't need to win. You've already won and now you're in bragging rights territory.
Focus 1/2 to at least 1 day a week on actively building a private money network. This is what really allowed me to grow my business beyond my yearly goals year after year. With good private money lenders at your back, what you can accomplish will surpass even your craziest dreams. Several years ago I wrote down some wild goals that I thought I'd surely disappoint myself with by December, but by June I was rewriting them because I had knocked 7 of my 10 out already all because of one private lender.
Every December spend some serious time thinking, really thinking about what you want to accomplish over the next 12 months. Put these on paper. I put these on a big white board directly above my desk. I have several paper copies as well. I keep one next to my bed and try to look at it before I go to sleep and once I wake up. I keep another tacked to the wall above the toilet paper in the master bath. As my good friend likes to say, "When I'm stinking, I'm thinking." I have my goals numbered and at the right side of the page a line I can write the date on that I complete the goal.
I could go on and on, but these are some of the habits I picked up along the way which I had in the beginning.