Research, research, research
Check the legal ins and outs in your country: find out how a long term let is legally defined in your country and be aware of any rights that long term tenants have. In some countries tenants automatically gain the right to stay in the property once they've been there for over a certain amount of time.
For example, In Spain a long term contract is for a minimum of five years, even if the contract states a shorter period. A tenant is legally within their rights to renew the contract once it expires for up to five years.
Don't forget your community rules: if your property is part of an urbanización or property complex, check that long lets are permitted under the community regulations.
Know your zones: there may be zoning regulations in place that regulate what rental activity is permitted in an area. Some zones prohibit lets of over 30 days to the same person. Zones vary between regions and neighbourhoods within a country.
Zoning is particularly prevalent in the US and is now being increasingly enforced within Croatia.
Find out your legal obligations and rights as a long term landlord: talk to a professional with expertise in property law in your country and ask fellow landlords via online forums.