Web-based so it's viewable anywhere, anytime on any device
Never include the price in the email. Always give it it's own section in the proposal itself.
The first thing they see when they open it should "WOW" them.
Your introduction should be personally written for them and address their real problem.
It should describe exactly what you're going to do for them.
It should include undeniable proof from previous clients that you can do what you say you can do.
Attractive and relevant imagery.
They should be able to sign it online.
It should be sent to your client while it's fresh in their mind.