(1) Don’t hire a parasite … I mean property manager
(2) Get to know some tradesmen (electrician, plumber, handyman) that you can call if your tenant has problems
(3) Vet tenants yourself. You need the kind of job where you can take a week or two every few years to check on the places.
(4) Tenant quality — grad students and self-employed are best. The former are likely to stay for a defined term and tend to be less rowdy than undergrads. The latter have every incentive to stay since they might have problems with other landlords being small-minded about non-W2 work.
(5) After buying the place, fix everything that might break in the next year or two. Replace any dodgy appliances, water valves, etc and so forth.