A fellow redditor made a basic form that you can add to and let your senators know that you are not in favor of this bill.  Read the information and learn about net neutrality, then send in a letter or email to your senator.

Dear <Senator or Representative Name Here>,

I am writing to express my concern over the “Internet Freedom Act of 2009” proposed by Senator McCain of AZ. Let me make clear – I do not support this bill.  I want the internet to remain truly free and Mr. McCain’s proposed act would be a huge step backward in that effort.

If you do not fully understand Net Neutrality, please visit with an unbiased person in the technology field.  Net Neutrality is a synonym for freedom in the technology community.  Mr. McCain is undoubtedly fighting against freedom this day, while using the name freedom to promote his agenda.

The way that you vote on this issue will unquestionably have an affect on if I vote for you.

Thank you for your help in this urgent matter.

Sincerely,
<Your Name>
<Your Address>
<Your City, St, Zip>
<Your Telephone Number>

The real net neutrality bill can be found here or here.
Senator McCain’s bill is here.