I've been working towards doing this independently - thank you for the community, it's a great idea. Here's advice on how to format effective letters:
http://www.ehow.com/how_2118434_write-proper-letter-legislators.html If we're really going to push to end the partisanship, then you might try this: reach out to someone you respect who has different views, and dig into their issues and concerns. Then put whatever common ground you can find into your letters.
I have a two friends who voted for Bush his first term (I did not, at any time), and I managed to find common ground on cost and cost effectiveness. That will be in my letter. As will a request that they focus on the important issues (economy, foreign policy, health care, medicare) instead of getting distracted by the culture wars issues (ie, gun control).