I'm a medical student taking final exams in a few weeks time and what I learnt is that to do a Valsalva maneuver, you keep your mouth shut and pinch your nose shut and exhale against resistance, feeling your eardrums "pop" if your eustachian tubes are patent.
So, by my reckoning, jkeak's writeup above is correct. As far as Segram's writeup goes, I think he is confusing intra-thoracic pressure with intra-abdominal pressure.
Most people do Valsalva maneuvers inadvertently when blowing up balloons.
The nitty gritty - doing a Valsalva maneuver causes a transient lowering in ventricular filling and thus reduces the intensity of most cardiac murmurs. There are two exceptions to this rule, these being the murmur of hypertrophic cardiomyopathy and the murmur of mitral valve prolapse, which paradoxically become louder during a Valsalva maneuver.
A Valsalva maneuver also increases vagal tone (parasympathetic) transiently. Harrison's principles of Internal Medicine actually classifies four stages of a Valsalva maneuver, these being I - onset of expiration, II - continued expiration against a closed glottis, III - end of expiration and IV - recovery and describes how the blood pressure and heart rate change during these phases. I won't go into the blood pressure changes but briefly, in a normal person, the heart rate decreases in phase I, increases in phase II and III and decreases in phase IV.
A sometimes useful measure (to cardiologists at least) of (one of the parameters of) cardiac function is the Valsalva ratio which is the maximum heart rate in phase II over the minimum heart rate in phase IV of the Valsalva Maneuver.