Those who already have, or are considering contributing funds will be interested in how your campaign is going, so keep communicating with them about progress.
This could be on anything from update on an added feature of your project, to how you have taken on board feedback, responding to questions or just thanking backers for their comments.
This is also the case when things go wrong. For example, when the Fkying Disco Ball project struggled with their Burning Man installation due to bad weather, they kept updating their backers on what was happening and thereby managed to avoid complaints and disappointed funders.
Finally, remember, there is an abundance of campaigns out there that have already been successful. Before, after and during your launch there will be lots of easy wins for your campaign by copying what has already worked for others