Using rental property as an investment is like any other business; to improve cash flow, you must either reduce expenses or increase income.  Increasing income is usually easier to achieve, but unless the residents are on a month-to-month lease, the only times you can increase the rent is when the lease expires, assuming the rental market will allow an increase.

Rent increases depend on market conditions but increases in the 4-6% range are usually common.  It usually best to increase the rent even if only a minimum amount each time the lease renews.  This way the resident is used to expecting an increase.  Also the owner avoids having to tack on a large increase when the resident has been in the property for several years at below market rent.  A large increase is more likely to cause a resident to move then several smaller increases.  It is also recommended to increase the rent when a resident request that they lease the property on a month-to-month basis.  A Month-to-Month lease is normally an advantage to the renter and a disadvantage to the landlord as residents normally expect to pay a little more.

When considering the amount to increase, figure what your market rent would be.  The increase should not be substantially higher than that of a comparable rental house in the market.  A comparable property would be a house that closely resembles your house in size, age, location, and amenities.

Despite your best efforts, some renters will choose to move away anyway regardless of an increase.