Is it possible to consciously increase or decrease your own heart beat?
Yes, you can decrease your own heart rate very simply by stimulating the vagus nerve in what are called 'vagal maneuvers'. If you stick your thumb in your mouth, wrap your lips tightly around your thumb, and blow on your thumb like you are blowing up a balloon, you will stimulate the vagus nerve and decrease HR. This technique is often used clinically, and is termed the 'valsalva maneuver'. The vagus nerve runs posterior to the oropharynx. Blowing on the thumb increases pressure in the chest cavity and stimulates a parasympathetic response. Another technique is carotid sinus massage, which works in a similar way, by modulating vagal tone. Many other techniques exist. Another is the result of the 'mammalian diving reflex', which slows heart rate in response to immersing the face in cold water. 



Vagal maneuvers https://en.wikipedia.org/wiki/Vagal_maneuver

Carotid sinus https://en.wikipedia.org/wiki/Carotid_sinus

Carotid sinus massage http://www.ncbi.nlm.nih.gov/pubmed/3985038

http://medical-dictionary.thefreedictionary.com/carotid+sinus+massage

Vagus nerve https://en.wikipedia.org/wiki/Vagus_nerve

Valsalva maneuver (see section on heart, where tachycardia means high heart rate) https://en.wikipedia.org/wiki/Valsalva_maneuver

WebMD on decreasing HR http://www.webmd.com/heart-disease/atrial-fibrillation/tc/vagal-maneuvers-for-a-fast-heart-rate-topic-overview