Location is always important and to add to other suggestions, I would recommend researching existing transport links and upcoming developments in your area. For example, in Brighton there are direct links to London, but the commute has been horrendous of late. However, there is a possibility of a new line being built to lessen the burden on commuter trains. This will no doubt attract more potential renters who work in London, but wish to live further afield. The line would also affect surrounding areas such as Lewes, Horsham.
Secondly, as other have mentioned, you must think about the kind of tenants you are wishing to attract and factor this into choosing your property. Do you want hard working professionals who are conscientious and rarely at home? These kinds of renters are more difficult to attract, so you will need to spend time looking for an aesthetically pleasing property and invest in higher quality appliances and furniture packages (moderated)
If you are looking to invest in a student town such as Bristol or Edinburgh, there will be no shortage of your people attending university to fill rooms. Yet, if you go down this route there are other considerations, such as potential lack of respect for the property. In this case, you won't need to invest to heavily in a pretty property or expensive furnishings. However, there will most likely other costs associated, such as repairs etc.