If you like a little mayo or mustard in your sandwich, but you don't like soggy bread, try spreading those condiments in between your meats and/or cheeses. This careful layering approach could not only help prevent soggy sandwiches, it could keep them from falling apart when you try to take a bite.
Other sogginess-inducing ingredients, like tomatoes, should go in the middle as well. Also try creating a butter and dried mustard barrier for your bread.
Both the Kitchn and the WonderHowTo links below will help you further perfect your sandwich-making skills.