People will back your project if they think it's worthwhile, but it's always good to have great benefit for your pledgers too.
You need to figure out what you can and cannot offer, and there are some restrictions you should be aware of.  You need to be fair with your rewards in terms of price points, and make sure that you can actually fulfill them.
It is fine to promise your pledgers big rewards, but don't forget that delivery can take considerable time and effort. Be realistic with your rewards.