A good proposal starts with a great template. Your proposal needs to be organized by sections and supply all the information the client requests. Create a template that works for your business, and you will be able to reuse parts of it for any future proposals. "Boilerplate" information includes a summary of your business, management bios, experience and past project highlights, and your financial statement.
Think about what makes your organization great. Is it your cutting-edge technology, or your super-qualified team? Make sure those areas in your proposal are highlighted. A proposal is like a resume for your business. If you want the job, you have to stand out. It's also important to include your best work project samples that are similar to what the prospective client is after.
Read the RFP thoroughly, and ensure that each and every section is addressed. If you have questions, ask them up front. RFPs always have a list of required items. Make sure that nothing is left off the list. If you know what the potential client is looking for, it's also appropriate to include samples of ideas you may have for them. Last, be on time! Never wait until the last minute to submit a proposal; let the prospective customer know you are eager for the work.