What a great question. The worst thing that can happen with a popular item like this is that you would get a soggy bread with soft and mushy tomatoes for the filling. That would not be very well done at all. Here is how to combat that:
Properly toast your bread. This is one of the single most important steps here. If you follow nothing else, follow this piece of advice. You need to have that toasted layer on your bread vehicle for your tomato layer.
Use Italian roma tomatoes. They pack a lot of flavor, and they are meatier and dryer than their beefsteak cousins.
Seed the tomatoes. You want to get as much liquid out of your bruschetta mix as possible.
Season just before service. This is important, as salt will draw the liquid out of the tomatoes if it sits. The best way is to adjust the flavor with some salt crystals right before serving.
Build the bruschetta just as you are serving them. Now, this would be a pain in the butt, but you will ensure that your canapes are not wet and soggy. Try not to make them way in advance, or you will get a mushy surprise.
Follow that advice, and you should have less problems with your mix. The same goes for bruschetta with any type of mix, such as anchovies, artichokes, cheese and other fillings. The fundamentals remain the same.