There is no one proper position of the sound post for all instruments. Each instrument has its own optimal position for the voice.

However, there are some rules that apply to all instruments:

• The sound post should have the width of half a bridges foot pointing towards the tailpiece and away from the bridge
• The sound post should not be positioned outside (toward the f-hole) of the imagined border of the bridges foot
• In general, the high strings become stronger when the sound post is positioned further outward while the lower strings are stronger when the sound post is closer the middle of the instrument
• It is important that the sound post never be moved while the strings are fully tightened as the cover could be damaged. Pictures of such damage can be viewed on the following photo.

Remember that the ideal sound post should connect to the belly and back and also have the correct length. Both factors change when the sound post is adjusted. It is best to go straight to a violin maker for this and to adjust the sound post by yourself only in an emergency.
New: How this is done by a violin maker and how you can do it yourself is shown here.