There’s another approach that I haven’t mentioned yet which I use every now and then, whenever I want to look at what exactly the browser is sending. Check out tamperdata, a firefox extension which can alter the requests from the browser. While its main purpose is presumably to alter the data (which, as an aside, can be very useful for testing server and server application behavior in many ways) it can just as well be used merely for observing said data.
Of all the methods I’ve discussed this one is the most convenient as there is really nothing to do beyond installing the extension. Whenever you wish to observe the interaction just open the tamperdata window (from Tools menu) and watch away. The limitation, of course, is that it’s only good for watching interaction originating from your single browser session. But often that is all you need.