Use a good drawing package that will locate the components you are using on the printed circuit board.
View your Gerber and Excellon files with a separate viewer, not just the one provided to you by your design package.
Consult with your PCB assembler to insure the finish you chose will work best with their assembly processes.
Begin by placing the components that require a specific location first.
Leave at least 100 mils between components and the Printed Circuit Board edge.
Attempt to space out your components evenly horizontally and vertically, and orient like printed circuit board components the same direction whenever possible. Insure that the orientation of polarized parts is the same. Avoid placing your components at angles other than 0 or 90 degrees
When it is necessary to have components on both sides, keep sensitive, heavy, or through hole components on the primary side. Also, any components that need special attention should be kept on the primary side of the printed circuit board as well.
When deciding where to place PCB assembly components, always attempt to minimize trace lengths.
Consider the volume of the run. If you have a low volume then PCB assembly by hand is a viable option. If you have a high volume run, it is most cost efficient to use automated printed circuit board assembly. Also, the volume is not only determined by the number of printed circuit boards ordered but also by the number of components on each PCB. Check with your PCB assembler to determine what volume your run is considered.