Tigersanddragons brings up a really good point. I don't use a notebook but what we do is to have a spreadsheet drawn up for each show, that lists all of our soaps & skin care products, along with some blank area's to write in any last minute additions.
For each product, we list the starting inventory, then we have an area next to the products where I put hash marks for each item sold. We also have area's where we can put hash marks for specials that we run.
The importance of this is twofold IMHO. First off, this allows us to total up our sales for the show to get the total sales number to calculate any tax that we might owe (sales tax). Some states we do shows in don't have sales tax but I still want to see those total sales figures. After deducting our booth fee, expenses, etc., we'll get to see our profit for that show. That helps to determine if we want to go back to that show in the future.
The other important thing, and this is really important, is that we have a record of what actually sold. We can see which products are more popular, which are not. We can compare this years totals against last years to see if we're making progress. But the main thing for me is that we get to see into the minds of our customers, to see what they like & want to buy. This helps us to plan inventory levels, as we'll double up the inventory on items we sold out of or that sold really well & this also gives us insight as to what additional complementary products we might want to develop.
It's also interesting to see how different products will sell completely different at different shows, different area's of the country, etc... All really good info to have for your business.