1. Determination: as pg always says that most successful people are those who're determined to bring their dream come true.
2. Code: somehow you have to manage to get the source code in your control. ,
3. Backup plan: make plan-b ready for damage control anticipated by your partners leaving project.
4. Speed: buddy, you need speed to roll-out to deal with competition.
EDIT: somehow you have to make an agreement with your partners (if possible do it legally) that if they decide to quit or if your partnership is dissolved then IP (code) remains with you (because originally the idea was yours) and in the future they cant do something similar either, i dont know whether it is completely enforceable legally but i dont think if any company (say goolge or yahoo or MS) will let me pursue the idea on my own that was originally not mine - so in the same way you can have an agreement or contract with your partners too.