If you're bringing a sandwich(es) to eat while travelling (the best option because it doesn't need a container or utensils), you have to build it to last. After all, you probably won't be eating the darn thing for at least three hours.
Bread For moist fillings--tuna salad, tomato--go for crusty breads, like ciabattas or baguettes. They'll soak up the flavors without getting soggy. Use sliced bread only if you're making a super simple sandwich, such as ham and cheese.
Mustard Condiments should be on the opposite sides of a sandwich; you don't want the flavors to get homogenized. To add richness, try a dash of olive oil or vinaigrette instead of mayo. Mustard travels well too--it packs a lot of flavor, and it doesn't spoil.
Cheese Don't be worried about letting cheese stay unrefrigerated for a few hours--most varieties are best at room temperature. Hard, aged cheeses are better than soft ones, which can melt or disintegrate. A slice of cheese also acts as an insulator between layers (to help control a tomato's moisture, for example).
Meat Most meats can't be trusted after more than an hour out of the fridge. Choose well-cured ones, such as prosciutto, serrano ham, and salami. Bacon, which is naturally preserved by its salt content, is another good option. Cook it slowly and thoroughly, to the point where most of the fat is rendered, so it stays crispy.
Fennel The minute lettuce touches something oily, like mayonnaise, it wilts. Fresh, thinly sliced fennel makes for a terrific alternative--its crunch doesn't sag with time. Cucumbers work well too, but they can be watery, so make sure your bread is crusty.
Paper Wrap the sandwich in light, thin paper--the kind used in delis--and slice it in half. Then wrap it in wax or butcher paper, which will keep moisture inside. The outer layer can be used as a makeshift plate.
Happy Travels!!
