they treat it like a business. as far as taxes go, there are deductions available for your expenses that are only associated with the rental property. you have to keep in mind what kind of tenants you are renting to. a lot of people also outsource the management of the rental property, so they hire a firm to take care of everything for them (repairs, collecting rent, yadda yadda). the firm takes a big cut of the rent and you just end up getting a check in the mail.
**edit** pat already responded...
i think what pat is getting at is that if you have to have a loan on a rental property, you are most likely going to lose money on it. if you can buy it cash, then you'll have a positive cash flow from the rental as long as you screen tenants and do a lot of repairs yourself. if you outsource it, then you'll obviously be cutting down on the scratch you take in.