Headspace gauges such as those made by Wilson work fine. But I prefer the Precision Mic that RCBS makes. It will measure to the 1/1000 of an inch and is avail in most popular calibers, incl the 30-06 and 223.
Put simply, headspace gauges help you set your sizing die so that your brass is sized only the mininum amount needed to function and fit your chamber.
Over sizing brass will lead to shorter case life. Also, by pushing the case shoulder back further than necessary, the case, upon firing, will stretch more as it reforms to your chambers dimensions. This is bad - the area where the brass stretches is just above the case head... can potentially lead to case head separation.
someone can do a better job explaining this than me, but this is a start.