Remind them why they need you. Your proposal should immediately remind the prospect of what their problem is and why they are talking to you in the first place. In other words, you should immediately start your proposal off with the pain your prospect is feeling. Are they not making enough sales? Is their message not getting out to their audience? Could their life_savings be in jeopardy? They are talking to you for a reason.
When you are writing your proposal, attention to detail is very important. You must write everything in clear and concise English. You should never use slang and only use the technical terms that you must use. The person who is reading your proposal may be a lay person and become confused if you use too many technical terms or job related slang words and phrases. Color for your graphs and charts is quite acceptable and as it eye catching can mean the difference between submitting a wining proposal and one that ended up in the "also ran" file.