Start off by being competent at maths I think helps and also to do it in person requires a very good memory. I wouldn't bet any size at all while practising such a thing, assume failure while you learn and of course the trick is old and looked for.
I have a degree in math and card counting requires elementary school math. (Counting up and down. Negative numbers, division and maybe some fractions.)I don't think this was my particular problem. Also, retaining a count in your head only involves short term memory. You do not need to be like the character in Rainman and keep track of the value of every single card..
Also, I just downloaded a simulator program, and according to my trial runs of several 400 million hand rounds, if my bet range was 25 to 100 dollars, the expected long term return was less than 1 dollar per hour.
Perhaps that was my problem. Many times, I would only bet between $5 and 25$. So the returns seem to be even more puny. Taking a "break" and playing another game like the slots, could have easily wiped out any returns. It appears that in order to get a decent return of 20.00 per hour, I would need to bet between $500 and $2000 per hand. I never had that kind of bankroll. Plus I am sure on the bigger limit tables, the heat one gets from the pitboss would increase..
That movie was a classic but more unrealistic in the world of the casino's actually beating up gamblers and not more or less kicking them off the property and not allowing them back.
I can only report two instances where I actually experienced heat in the casinos. In one, I upped my bet from 25.00 to 50.00 and the pit boss came over and looked at all of the cards in the discard tray. I knew what he was doing, so I left. The second instance, the pit boss simply pushed my bet back and stated that I could continue to play any other game but blackjack. Now that I have an idea of the expected returns from my card counting, it was probably of the least concern to them. Especially when they have other players on the table not playing the basic strategy.
I once had a player who was playing the no bust strategy and yelled at me because I hit a soft 17 and was "screwing up the cards."