sensors for detecting breathing movements
There are two main methods you can use -- impedance plethysmography (http://www.bem.fi/book/25/25.htm) or a simple belt transducer (http://www.thoughttechnology.com/sciencedivision/pages/products/respiration.html) -- but search on "respiration belt sensor" for a variety of methods to do this.  Some use strain gages, some use pressure in something like a cuff.  The first time I've ever seen this described was simply a rubber tube filled with mercury, but you don't want to do that!!