This is pretty much the only way I have had good luck buying properties at a discount. Once a broker, commission and of course public competition is involved the price almost always gets pushed up immediately (low hanging fruit). We will buy from a broker of course, but usually end up paying more unless it is a discount broker with a set 10,000 fee for example. The only exception is when we jump in quickly with a solid offer and PA agreement before the market has a chance to respond....most sellers know this and let the market respond though.
I network with brokers quite a bit and make it clear we are really looking at pre-market deals mainly. You will be surprised at the list that comes up, of course being able to perform is key to gain access.
We use Costar to look at existing properties that are not on the market (you can view any commercial property). It is expensive, but you can pretty much decide if you want to buy a property before even making a call to the owner. I haven't done a lot of this, but blind offers are an option....it is hard to ignore a piece of paper that has an actual offer on it for your property....takes more time to put together, but probably will increase percentages of responses (hopefully not negative) significantly.
We also use a few brokers to make calls to owners for me and pass call on to me when it makes sense for a set fee. This is a great way to cover a lot of ground and avoid having to make all the calls yourself. A lot of times I will identify a list and then have them make contact. I am a broker myself, so I have debated on putting a team of RE agents together, but like not having that liability by using other brokers instead.
I think the shortest form of contact is best (in person is more effective than calling...calling is more effective than a personalized mailer......personalized mailer is more effective than a general postcard). I get mailers all the time for 103 or interested buyers etc. Most of the time I toss them, but if they are good I create a template to modify and use myself, but honestly have stayed away from mailers at this time, so I can't say anything either way on their effectiveness. I might have to give it a shot :-).
There are some other techniques...that probably are not as common lately such as software that shows properties that are behind on taxes or mortgage payments, those are usually good owners to contact if possible, this is my plan when the market slows down again.
I am surprised no one mentioned banking relationships, I was working with a group that bought 700 units this way. Banks see the whole deal and when it is suffering will often offer it to the best party to manage during receivership and often sell at a discount to move things along. I always know it is a good call when a bank reaches out to us on a property that has run into issues.
Overall the best strategy is to network like crazy, show a great track record and keep looking even when it looks hopeless (you should see what people are paying in our market right now....it feels hopeless....but I am going to take my own advice).