There are two main factors that are essential in the growth of tourism: Access and accommodation. For a resort or city to become a popular tourist destination it must be easily reached and have large numbers of beds providing a range of accommodation.
Another obvious answer is attractions: People need a reason to go somewhere. An obvious exception to those rules is Las Vegas but having been virtually created out of nothing it has the airport access and thousands of hotel beds that has turned it into an attraction on its own in the middle of the desert and brought in supplementary businesses that want a slice of the action.
A natural infrastructure that makes a place reachable by car, train and bus helps to draw in tourists, this is especially true of most European cities where long established transport infrastructures makes travel easier. Where that infrastructure isn't in place it has to be created with the likely market for the area key to deciding how transport/access should be created.
Providing a range of accommodation is essential to retain visitors and to encourage them to extend their stay. Depending on the target market accommodation will be designed accordingly. A business or conference destination will be require three and four star hotels to attract visitors whilst a bag packers type place is more in need of hostel type accommodation of the cheap and cheerful variety.
Having a coherent strategy towards tourism also brings focus to an area. Knowing the aims, attractions and market that you are looking to attract makes promotion and marketing fairly straightforward. When all these factors are in place, when research is completed and visitors' views are sought and monitored the tourism success of a place can be managed and developed.