I have lost track of the size of the pot a few times since coming to Vegas. The cocktail waitresses are usually to blame.
I have already gotten pretty good at looking at groups of 5 $5 dollar chips and counting the apparent number of groups. It is faster than actually counting the pot and it has wound up being accurate.
All of the advice about counting it as the money goes in, is, of course, preferable to having to estimate, but if you do lose count, try to break the pot down into groups of chips and count the number of groups. It has worked for me so far.