Warning: preg_replace(): The /e modifier is deprecated, use preg_replace_callback instead in ..../includes/class_bootstrap.php(430) : eval()'d code on line 456 Pls help to translate to Eng - Thanks in adv!
I think that saying "in advance" is unecessary. Just say thank you, or thank you for your consideration of this matter (although I think just thank you is best.)
If you are requesting something that has not yet occurred then it is obvious that you are thanking them before the actual act or event. I say "Keep it simple".
Anyway, if you really want to say it, then "thank you in advance" is the correct wording.