Renters keep leaving early

[Brett Malen]

So I have 1 rental with only 2 renters, where the first lasted 11 mos. the second 2 mos. Both so far have been high quality, but effectively use my well maintained rental in an A neighborhood as a staging area to look for a permanent home. Rent rate is always in line with market. For the second tenant I had a lawyer draft an Early Termination Fee clause as the then prospective tenant casually mentioned he may not stay the entire 12 month term. Now the tenant doesn't want to exercise the ETF by giving me 30 days notice, rely on landlord exercising "reasonable effort" to rerent before the next 1st of the month and basically seamlessly walk away w/out having to pay fee or additional rent before the next tenant moves in. I have explained the 2 options for the tenant on multiple occasions 1. You can pay to leave early or 2. You pay until a new tenant is placed. Both of these are signed and in the lease agreement. I am fine with not receiving the ETF, but I will hold him to point # 2 per the contract. 

My questions to the community:

1. How would you manage this going forward to ensure or increase your chances of having the tenant stay the full 12 months?

2. What reasons can you think of for the tenant to find it in their best interest to pay the ETF and move on other than not having to pay for the duration of the contact?

Thanks!!!