Space Travel and exploration is expensive, however, the benefits and understanding gained are extremely valuable to humanity.
The United States used to lead the world in space technology. Due to long term cuts and weakening of our program, we now hitch rides via Russian space craft.
A voluntary tax listed within tax forms to allow taxpayers to directly fund NASA (like contributing to the presidental campaign fund) would go a long way in allowing citizens to support NASA. As this would be completely voluntary, there is little downside.
Additionally, provisions should be built in to ensure that this funding acheives the following:
1) All funds collected must be directed to the NASA budget
2) These funds are to be in ADDITION to current funding. The current budget must be maintained