If a property meets your criteria, then pull the trigger.

What strategies do you use to lock them up ?

I do not have a lot of money for an earnest money deposit, so what should I be saying to my sellers to lock up the contract quickly? 

Put electronic locks on entry door to house, and each bedroom door. you can easily change the door codes when someone vacates..

Issue can include land locked vacant lots, condemned or demolished houses, other government liens popping up, flood damage, fire, etc.  

It was the push I needed, so now I'm pulling the trigger for my first rental property.  

The areas you mentioned having interest in definitely have some two unit properties, but not  a lot so I suggest having your financing and analysis ready to go so that you can pull the trigger when you find one that fits your criteria.

My impression is that you charge extra rent, put it aside, and allow them to use that as part of the purchase if they pull the trigger in a certain amount of time.

After paying that off in eight years, I started looking again but the market was crazy and then crashed so I sat on the sidelines, saving and kept looking for several years before finally pulling the trigger on a six-family building in Sunnyside - I put $300k down, and used a cash out refinance on two other properties for the other $550k.

Many different looks but it is backed with a 15 year warranty and it is a vinyl plank with 2 sides of adhesive almost like a snap and lock.