I have a simple formula I apply to calculate the percentage a rent increase should be on a unit.
First, look on craigslist/zillow for other advertised rentals similar to and nearby yours. Take the average of what you find to determine approximate FMR. Then, take your FMR figure, throw it away, and use the formula below:
New Rent = Current Rent x H
where the variable 'H' = 1.0 + the headache factor for that tenant.
In all seriousness: I believe the #1 most successful rule for managing rental property is to find amazing tenants and then keeping those tenants as happy, long-term tenants for as long as possible.
I bought my first property nearly 10 years ago now. Have 3 units. I have only ever had one tenant leave in those 10 years, and that is because he was military and was transferred.
I have never raised the rent on any of my tenants because they are all taking care of their homes, paying reliably, and never cause me undue 'headaches'. And I've no doubt each of them will recommend me to their friends/family as a good landlord when I buy my next property - word of mouth is the most cost-effective marketing strategy there is.
Could I probably be grossing another $50-$100 now per unit? Maybe. But how much have I saved over the past 10 years in avoiding vacancies/tenant screenings/repairs/etc just by keeping the great tenants I have now?