A good RE lawyer and getting to know as many agents as you can. Good deals are rare on the open market. It is all about who you know and who they know. It took me two years to get to the point where people were calling me with good stuff.
If you are a buy a hold guy I would also not waste your time with wholesalers unless you know they are good at what they do. I can't tell you how much time I have wasted talking to these bottom feeders.
You also have to be tough in this business. This does not mean you should throw out the golden rule, but you might have to get close. You will start to walk into situations thinking, "How is he going to screw me over." This is fine and normal, but try to remember to keep a smile on your face (I have trouble with this).
If you are really going to get into rentals and self manage, you are going to have to really work. To get the good prices you are going to have to buy the junk others dont want. These will require repairs to make them into property that everyone wants. You will have to learn construction and fast. You will also not be working a 9-5 job. I work seven days a week 10-14 hours a day. I am starting my fifth year and still working this hard. You have to remember that it is great to hire people, but they will not work as hard as you and every penny you pay them is a penny out of your pocket.