A reseacher conducted the experiments on my subject and I would like to use his data on my thesis. I am a modeller so I don't do any experimental work and just to want to model his experiments to validate my models. We met in the past at some workshops. What is the etiquette for mailing for this purpose?

Model validation with other people's data happens a lot - perhaps couch it in terms of a collaborative journal paper after your PhD?

The main thing is make it clear that they would share authorship if anything was published as a result.

I think the other respondents have the right idea - you have to let the person know that you will consider co-authorship (and as it is his work being the basis of it, there would be a good case for him to be primary author ... again, you'd have to balance that up versus not getting the data at all). Get your supervisor on board and quite possibly have a well-written plan as to how you will use this information and why it is then publishable (giving possible journals and previous similar papers) . Pass this by your supervisor, who should then contact the other person with suggested journals (obviously DON'T give out all of the results and ffs, keep it documented). Have heard of cases of research stolen on basis of conversations, so even an informal document can act as your Proof of concept. Main thing is get your supervisor to ask as it would look a whole load better (could always prepare the stuff and then CC him/her).

Worst case is that the other person says no, publishes his work and then you use that paper as the basis for yours, either developing or disproving the earlier one.