Hi,
I'll just repeat what I have learned through trial and error:
1. travel on the weekends if you can - weekdays cost more.
2. try to make reservations as well in advance as you can - don't call the day ahead of time.
3. look into roundtrip fares before you even mention one way.
being spontaneous is nice, but you will pay through the nose.
I do not have the direct phone number for Eurostar on me right now, but if you are going to be in London for a while (one or two weeks in advance), call as soon as you arrive to make plans - you can pick them up at Waterloo.
I do not know, but I imagine you can also pick them up at the station even if you reserve on the website, which would obviate using a US address for mailing purposes...
