Are there any other expressions for “Thank you for your support” apart from “Σας ευχαριστώ για την συμπαράσταση”? Any other particularly colloquial expressions for this very important phrase?
Σας makes it pretty formal, but maybe that's what you're after.
Ευχαριστώ για την συμπαράσταση / Ευχαριστώ για την υποστήριξη are both valid options.
On the street, you may also hear Ευχαριστώ για το σαπόρτ, or even Θενξ για το σαπόρτ…
Depending on the level of gratitude you want to express, you might also consider something along the lines of Με σώζεις! / Τι θα 'κανα χωρίς εσένα;(But at this point, I'm just dropping more or less random ideas…)