Here's how I do:

1. Buy your tickets with a milage-collecting CC....such as AmEx or DC,
2. ....online, giving your smiles or maximiles number (syndicated national French fidelity-thing);
3. sign up for "voyager frequent" http://www.eurostar.com/FR/fr/leisur...travellers.jsp (sorry, French only) and once completing the trip, send in your tickets to collect credits with the Eurostar FT program.

While (1) is obvious, one is still collecting miles/points three different places at once. Not sure if one is supposed to collect maximiles/smiles and EuroStar credits concurrently, but it has worked for me for my past trips.....if there's anything better, let me know