A great template is the Heilmeier questions[1]. I will read and respond to any cold pitch that answers them.
[1] http://www.design.caltech.edu/erik/Misc/Heilmeier_Questions....
[1] http://www.design.caltech.edu/erik/Misc/Heilmeier_Questions....
The email should have a good subject and opening 1-2 sentences which piques her interest enough to keep reading. If you start by clearly and succinctly saying who you are, what you're doing (without any jargon or buzzwords), and why it's a fit for her firm, there's a good chance she'll continue reading a page long email.