So essentially your primary job is not a real estate investor, it is rent stalker. My gentle reminder is a 3 day notice to pay or quit. Either they get with the program or they don't and find another place to live. As my good friend and fellow investor says "just don't park your car with the oil leak in the driveway."
I don't understand why people coddle their tenants. The tenant's job is to pay rent on time and keep the place clean. After the first 3 day, they'll straighten out. If not, send them a letter and let them know you're increasing their late fee. I like to start at $100. If they are continually late, send them another letter and raise it up higher. You can even do a daily rate if allowable in your state.
Just for the record, I have one tenant who has been in one of my buildings almost 20 years, another tenant in a different bldg just hit 18, and his next door neighbor is at 15 years, and I just sent them all letters raising their rent like I do every January. Out of 45 doors, I have 1 vacancy and that is because I just purchased the property and finished rehabbing it.
I don't make friends with my tenants. I try not to ever talk to them. I am more than pleasant when I have to have a conversation with them, but I'm not here to buddy up with them and be all chummy sending them pleasant reminders. I just want the rent deposited into my account on the 1st of each month and for them to not bother me. I tell them all at the lease up that my goal is that they never see me again until the pre-move out inspection or at eviction court. Trust me, they don't want to see you either. A tenant's perfect world is to never have to deal with the landlord.