Is there any way to pressure textbook publishers to reduce price? I am an instructor in a medium-sized state school in the USA. The price for the textbook to the introductory math course that I teach just increased. Again. A new, paperback copy costs 170 USD. I want to put pressure on the publishers to reduce their price, but I am not sure what the best strategy is. 

Has anyone had any success in such an endeavour? Any ideas? A strongly worded letter, signed by the faculty?
If you don't have the authority to change the textbook on your own, at least for your own section, then your target should be, not the publisher, but the person or committee that does have that happy power.*

Show them this: http://aimath.org/textbooks/approved-textbooks/ and propose that you teach a section of your course using the appropriate open access textbook.  (Edited to add: Pick out the appropriate book and an alternate before you talk to the committee or coordinator and be prepared to defend your choice vs. the approved text.) In the following semester, compare how well your students did in the next course in sequence vs. those who used the standard text.

Your secret weapon: work like hell to be sure your students are well prepared for the next course.

In the comments, Ben Crowell has said: "Here is a catalog I maintain of free books, including many open-source textbooks: http://theassayer.org"  I've edited it into my answer so it doesn't get lost.  Thank you, Ben.

* Hat tip to Professor Severus Snape for "happy power."