It is very much a psychological phenomena. At the same time SWIM
started being more worryful and concentrate more on his body, he started having palpitations. He had been smoking as much before but there was a phase when he had horrific palpitations almost every time. Firstly the pulse will be accelerated if you concentrate on it and don't want it to rise. Secondly, it's very much depends if you pay attention to it. Your heart can beat pretty fast without you noticing if you just go with the flow.
I've noticed that my palpitations were bad when I was sitting or lying, but getting up helped. You're brain uses a lot of energy too; if you let your mind run wildly it can help similarly. But ofcourse thinking about very exciting things raises your heartbeat.
You could also try magnesium. It helps relaxing and lowering heart beat a bit. Beta-blockers are better, but they are under prescription in Finland at least, though not in many mideuropen countrys.