House Committee on Appropriations post on the subject
Write your own letter to Congress (site via the National Education Association, but you'll be writing your own content and can opt out of sending a copy to the NEA or getting on their email lists)
Model letter on the subject
Please feel free to use my letter, below, to model after for a letter to Congress. Also feel free to repose elsewhere.
I am writing today to strongly urge you to reconsider cutting appropriations for the James Webb Space Telescope (JWST) from the FY 2012 appropriations. I am a full-time Associate Professor of Physics and Astronomy at XXX Community College, and I live and vote in your district in XXX.
With the ending of the Space Shuttle program, the US is already falling behind its international competitors and collaborators in the field of space exploration. Our two most important space observatories, the Hubble Space Telescope (HST or Hubble) and the Spitzer Space Telescope (SST or Spitzer, which operates in the infrared as will the JWST) are soon going to come to an end as well. Without the JWST to replace them, we will be even further behind.
These space-based observatories are able to observe things that no other facilities in the word (or orbiting above it) are able to do. Observatories on the ground are not able to make as sharp images or images in the infrared due to the obscuring nature of the Earth's atmosphere. Other space observatories are designed to fill different roles than Hubble and Spitzer, and the work those two space observatories perform cannot be replaced by other existing space observatories. Once they are gone and without a replacement observatory, there will not be a single nation in the world able to conduct astronomy of this sort.
In addition, all the parts of the JWST have already been manufactured and are now in the process of being assembled. At the very worst, I urge you to set aside enough money for the project to be suspended and the parts stored for later assembly while investigating the management of the project, rather than axing the JWST entirely which would necessitate disposing of the parts.
Thank you for your time reading this letter. Once again, I live and vote in your district, and I will be urging my friends and colleagues to write to their representatives as well.