Writing winning proposal

Never title your proposal "Proposal". 

That doesn't say anything. Clients can't figure out themselves. Instead, use a benefit statement as the title.
Focus on your clients' business needs or mission objectives first: 

Mirror what you have heard from them before offering a solution. That addresses what they care about the most, and it shows you've listened and considered their interests and are not offering a canned approach. Do not have lengthy corporate histories in your proposal. Nobody's really interested in it.
Avoid using jargon: 

Even if your contact at the client organization understands all of your jargon, others who might be reviewing your proposal may not understand it (decisions are seldom made by a single contact). However, it is good to use client's jargon.
Keep your proposal as short as possible:  

You may be tempted to throw in anything and everything that might be of interest, but decision makers really won't read it. At best, they'll skim through your document. Shorter the proposal the more likely it is looked at first, which means all others will be judged in comparison to it. That's an added advantage if you've done a good job.
Highlight your key points: 

Since the decision makers skim, make your document more "skimmable" by highlighting the main ideas. Use bullets, headings and sub-headings, boldface type, color, borders, graphics, and anything else that will make your key points jump off the page.
Quantify your benefits and payback: 

Show the decision-maker how much she or he will save, or how much more productive the organization will be. A convincing calculation of your client's return on investment is more compelling than a slogan or cliche.
Prioritize your uniqueness factors or competitive advantages: 

Think about what you have to offer and select a few qualities, prioritized in terms of what your client cares about. If possible, tie them to that ROI you're demonstrating.
Ghost the competition: 

If you know who you are competing against, raise issues in your proposal that strike at their weak points. Don't disparage them or mention them by name. But if you know their product has longer implementation time, make a big deal about the importance of shorter implementation time.
Ask for their business: 

Ask for it in the cover letter, ask for it in the Executive Summary, and ask for it when you deliver or present the proposal. Being passive doesn't work. You have to ask.