[sexypanda]

I'm assuming you mean in a live game, since online it's usually right there on the screen for you. In a live game, you really have to make an effort to keep a running count of the bets. Keep this count in terms of number of bets rather than the actual amount of money in the pot, this makes things much easier. So just count the number of small bets pf and on the flop, then divide by two and add the big bets on the turn and river. If your conscious about this for a few sessions, it'll become second nature to you.