Also, remember to keep tracks as short as possible (unless you are using controlled impedance tracks) and as straight as possible without breaking 90/45 angle rule. Also, 100 nF ceramic + 10 uF tantalum electrolitic capacitors in large quantities. Especially the ceramic ones, stuff them wherever u can, since they cost next to nothing. As it was mentioned by leo_o2 fill all remaining space with ground planes. If your board is 2 layer then remember to place multiple vias just to connect ground planes in various places (like 1 via every 4mm). If your board has more than 2 layers use middle layers for power and ground. In this sint' critical in your application try to lower signal rising times by placing series resistor on signalling lines (like 100R or so).
There is a lot, lot more of that stuff.