Knocking on doors can work.

Unfortunately the last tenants left the bathrooms in bad shape, so the next owner will have to at least redo them before renting or moving in.

Ask them what needs to be repaired, when was the last time the kitchen was updated, when was the last time the bathrooms were updated, how old is the roof, does the house need paint. 

What if I'm to leave a simple yellow letter on someones front door?

But then again retirement is still many, many years off for me, so I'm thinking my best strategy may be to bet on Southern California appreciation for the long haul...I don't plan to quit my job any time soon, and I'd take 1% greater average year-over-year appreciation over the next 30 years over an extra couple hundred bucks monthly cash flow per door by picking up property in Cleveland or Indianapolis...decisions, decisions...Let's say you were in your 20s and could do it over again.  

You can do a little work yourself and get a list of properties that fit your specs that are absentee owners from listsource and then either knock doors or conduct a direct mail campaign. 

What is your objective for door knocking? 

What I do is search by bathrooms.

Of course, it has to be in your brand new-car, and you have to pay the gas and probably get this investor to lunch, after this you may have to write 30 offers,  all way below asking price, and if you are lucky enough, you put one under contract, then you have to go with your buyer to inspections and very likely you have to open the door to 2 or 3 contractors so they can start doing an estimate.

Direct mail, door knocking, cold calling, craigslist ads, word of mouth, instagram, facebook, twitter.