Hi Bjoern,
Check this article out,it gives a good explanation on working with acrylics:
http://www.acrylicosvallejo.com/blog/2010/03/painting-figures-with-model-color/
Also check out Timelines and planetFigure, there are some incredible examples of painting with acrylics.
Tamiya acrylics aren't the best for hand painting. Try Vallejo, Reaper or Citadel(Games Workshop). You'll notice the difference.
Russ