1. Craigslist has become the most popular way to advertise your rental properties.  It is fast and free.  You can place pictures in your ad and be as wordy in your descriptions as you like.  Plus you can interact with your prospective tenants via email instead of by phone to save yourself time and be most efficient.  Prospective tenants can search for ads with keywords that are important to them like hardwood floors, garage, or by city like Saint Paul houses.
2. Newspapers were previously the only way to advertise your rental properties.  In some communities, this may still be the case.  Newspapers typically are expensive, have limited space and no pictures.  Running your ad for multiple days will yield the best results.  Only running the ad on Sunday or Thursday may seem like a less expensive way, but newspaper readers often will skip days or simply pick up a paper when they have a need (like looking for an apartment).
3. Put a sign in the yard.  In some areas, especially urban and college areas, this can be an effective and quick way to advertise your vacant apartments.  This choice will require you to take and return calls from prospective tenants so be prepared.  Alternatively, you can find a voicemail service in your area and record details about the apartment for prospective tenants to listen to.  They can then leave a message if they are interested or attend an open house at the property at a preset time.
4. Other rental services exist that will advertise your property on-line for a small fee.  Many of these are very effective, but trial and error is best.  Some services will have better reach with a certain type of property or area.

Finding the right tenant up front help prevents you having to do an eviction later!