How can I stop chocolate chips sinking to the bottom when I make cakes?
Coat the chocolate chips in flour (whichever type of flour goes into your cake).  Put a few ounces or two of flour and the chocolate chips into a zip-top bag, close tightly and shake.  The coated chips will then adhere and tend to "float" in the batter.  Subtract the amount flour used to coat the chips from the flour otherwise used in the recipe.  

You could do this with cocoa powder instead of flour.