The Standish Group performed a study (The Standish Group, "Unfinished Voyages" (1996), http://www.standishgroup.com) on factors which contribute to the success of an IT project. These factors were subsequently ranked with respect to relative importance:
 

Success Criterion	Relative Importance
User Involvement	19
Executive Management Support	16
Clear Statement of Requirements	15
Proper Planning	11
Realistic Expectations	10
Smaller Project Milestones	9
Competent Staff	8
Ownership	6
Clear Visions and Objectives	3
Hardworking, Focused Staff	3
Total	100

 
The results of this study go to suggest that project success is most likely to be achieved through involvement of project stakeholders, support of the project at an executive level and clear statements of requirements - Some pretty interesting stuff to remember when working on IT projects.

Update - Information about the Unfinished Voyages study can be found at http://www.standishgroup.com/sample_research/unfinished_voyages_1.php