Keep the project focussed - Clearly state your objective and stick to that. Do not add unnecessary or tangential material which will only confuse the reviewer. Less is more - Do not make the mistaken assumption that several projects are better than one or have a better chance of being funded. The reviewers will react negatively to being presented with a patchwork of unrelated projects. Communicate with the Funding Agency You should learn all you can about how the funding agency works, what their review process is and what constraints they operate under.
Most organizations encourage proposers to talk to the responsible person.
NSF and NASA program officers and proposers alike say that this may be the most important step a new proposer can take. Learn the system - Always take the opportunity to review proposals or to serve on a review panel when you get the chance. Panel service in particular can be very educational. There is nothing like reading thirty proposals to help you understand how a good proposal should be structured and written. Understand the timing and constraints - Try to get a feel for when the best time is to submit a proposal for the particular organization.