Do your research
Dodgy neighbours; flooding from a creek running behind the property; or just a change in an area's demographic make-up can all be reasons why a property is on the market. Local shops are usually a great source of handy information. Take the time to go and chat. Enquire at the local council if there are any development applications in the area or environmental issues that could affect the property. Check local crime statistics too.
Never make an offer on a property until you have taken steps to determine its true market value, and how viable it is as a rental investment. Research recent sales values and current weekly rents in the area, and make enquiries about the tenant history of the property you are interested in.
If you are satisfied that everything stacks up, you can start your negotiations with an offer that is 10% below the asking price. However in a sluggish market it can be worth pitching your first offer at 15% below the listed price.