thank you very much in advance (for your assistance o.ä.)

We'd like to thank you in advance for trusting us to use your invaluable material in a responsible and productive way...

That's a bit more formal.

Please send me .......
Thanking you in advance,
yours sincerely

When I was at school we were taught the phrase "Thanking you in advance" to use in letters
Hope this helps

Diese Abschluss-Phrase kenne ich aus Briefen/emails von amerikanischen Kollegen, allerdings nicht unbedingt aus England.
Ich benutze meist "Thank you in advance", habe das aber unter einem englischen Brief/mail noch nicht gesehen.
Da heisst es meistens "Thank you for your co-ordination." (Ich weiss übrigens nicht genau, ob der Bindestrich sein muss, manche schreiben es so, manche ohne.)

@Heidrun: I'd agree with your comments. The "advance" thing is not common, and your suggested translation hits the right note.
But methinks you mean "cooperation", not "coordination", with or without the hyphen

@Ghol - your are certainly right, I meant cooperation! And I prefer whithout hyphen...

Ghol,
> The "advance" thing is not common

I am confused, as usually one can trust your comments. However, there are over a million Google hits for "thanks in advance", there even is a common abbreviation for it (TIA), I get e-mails holding this line frequently, so why do you believe that "the 'advance' thing is not common"? It is probably not formal enough to appear in a written letter, but in my experience at least as common as "Your help/assistance is (very) much appreciated".

support Sophil for comments . it would never occur to me to use anything _else_ .

an appreciation of someone's efforts is something i'd direct _afterwards_

and thanks for co-operation need have nothing to do with the thanks being in advance.

The hyphen is somewhat old fashioned, i assume. I never saw it until i read Raymond Chandler who published in the late 30s.