This from the New Hampshire Lakes web site:

"Here are a few tips you can follow to make sure that you have a great time on the ice this winter:

Check it! Never assume the ice on any water body is thick enough to support your weight.  Start at the shoreline, and, using an auger, spud or ax, make test holes at intervals as you proceed away from the shoreline.  As a rule of thumb, for new, clear ice, there should be a minimum of four to six inches of ice to support a few, well-dispersed people; six to seven inches for small, on-foot group activities; and at least eight to ten inches for snowmobiles.  (Ice thickness recommendations are based on information from the Cold Regions Research Laboratory in Hanover, N.H.) "

Ever wonder how a lake actually freezes?  Read this article from the New Hampshire Lakes association that describes the lake freezing process and answers some of those nagging questions you've always had about this process.