This article from Lawpack's Residential Lettings Kit discusses the most common methods of finding potential tenants:
Newspapers and magazines
You should choose the paper most likely to be read by your target class of tenant. Usually this will be the local paper. Most local papers will have a particular day in the week when local property is advertised, and may have a property supplement. However, sometimes a different paper may be appropriate. For example, holiday cottages are often advertised in the Sunday papers or in glossy magazines. If your tenants generally come from a particular large company in your area, they may have an in-house journal where you can advertise.
Shop windows
This can be a cheap way of advertising a property if you are looking for a local tenant. Most newsagent shops offer this service.
Notice boards
Perhaps the best example of this is university notice boards, if you wish to advertise properties for students. Be warned, however, that many student unions will only allow landlords who meet certain (often stringent) quality standards to advertise. Local businesses may, if you provide accommodation regularly for their staff, allow you to put a card on their notice board.
The internet
This is especially useful when looking for student tenants, who will often have free internet access through University. It is the fastest growing method of advertising your vacant property and looks set to become the most important.