This app note discusses common problems encountered when starting designs involving AVR microcontrollers:
This application note has been written to provide answers to some of the questions and problems faced when starting designs involving Atmel®AVR® microcontrollers.
The application note treats topics that are known to cause problems. The scope is to provide an introduction to potential design problems rather than being an exhaustive walk-through of how to design applications using the AVR microcontrollers. This document is thus a collection of information from existing Atmel AVR documents, combined with information that is not previously documented.
View the PDF document for more information.
It is funny when you look at the layout example (page 15), then you notice that the vias of the power-plane connections are between the capacitor and the cpu pins. I always learned that you need to put the vias on the other side as to keep the cpu/capacitor line exclusive for the high current path.