While it can be costly, these older buildings can be brought into the 21st century with relatively sound energy efficiency and modern amenities.  

Are you going to be making big changes to the building envelope to increase comfort and energy efficiency or are you just doing the minimum to flip it. 

Furthermore, I also have spent some time sifting through these publicly traded entities reports and some of these guys are operating significantly less efficient than I am (such as 20%-30% vacancy rates).  

Don't change your air filter frequently enough and you reduce the life of the unit, not to mention its efficiency. 15 min here or there could make the difference between hundreds or thousands down the road.

That leaves management with the task of decreasing expenses, run it more efficiently, but in reality, there is only so much you can do here as a 2-4 unit isn't at an operational level where expenses can be significantly cut.You can't squeeze blood out of a turnip.Chances are, a savvy owner isn't going to be leasing you a property for you to let to someone again at any killer earnings potential, not much more than management fees.

Is it by being a tough negotiator, an efficient rehabber or a savvy marketer?

My plan would be to add value to the property by spending 20-25k finishing the garage and adding another unit - most likely an efficiency ($575/month).  

They were prompt, efficient, and accurate.

For the most cost-effectiveness & efficiency, does one usually purchase materials needed & either have the contractor pick them up or delivered to the site?  

We have also found that tenants will pay a little more rent to have energy efficient laundry machines in their unit.