So iSkat has launched and after 3 days we are around position 30 in the overall AppStore (Top 12 Games). Since we do not have a considerable budget for Marketing, we consider this to be a great success for us. Yes 
How did we do this? Unfortunately we havent found a magic trick to boost sales Still the best way to boost your sales is to have a great app. It certainly helps if you have an app people search for. I guess there are a lot of Germans out there who just search for "Skat" on the AppStore and find our game.
However it helps if you marketing and therefore here are some useful links I found:
Also very interesting is this presentation: