For me the trick to not coming home with any new pets (and this ESPECIALLY applies to herp shows) is to make a list of EXACTLY what you plan on buying, then going right in and buying it, then leaving. Another thing is to think of all the costs associated with a purchase above and beyond the cost of the snake (or lizard or tarantula or whatever). More food, more heating elements, another viv (or shoebox), hides, etc., etc. Plus the space requirements.
Not that it works, but if you come home with a new pet anyway, at least you can tell yourself that you tried.