There are two core tenets to the assembly of any sandwich. This only applies to true sandwiches (see other posts for the definition of a sandwich). Both should be balanced and neither should be used exclusively.
Maximize Inter-ingredient Friction
The one truly unforgivable act in sandwich making is creating one in which the contents slip out (anyone who has had cheese, lettuce, and tomato adjacent knows the consequences, especially if they’re greased with a condiment). Generally, flat ingredients with low friction should be separated. While a large surface area to volume ratio is great for taste, condiment distribution, and other reasons, placing large flat surfaces next to each other is risky. Layering ingredients intelligently can literally make or break a sandwich. Additionally, the same ingredient can be used in multiple ways. For example, cold cheese slices can act as an ice rink for your roast beef while hot cheese can fuse a greasy hamburger patty with a flat slice of lettuce. Note: consider dicing troublesome ingredients such as onions and lettuce so they can be better used as a composite binder rather than a slip-n-slide.
Stack Ingredients From Least to Most Healthy
Assemble sandwiches beginning with the least healthy ingredients at the bottom. The bottom of the sandwich is the first thing that comes into contact with the tongue, the primary location for taste. Therefore, whatever the eater you want to taste the most should be closest to the tongue. The sandwich maker can take a slightly different approach to this in which a given ingredient needs to be used more or less depending on its location within the sandwich. For example, in order to give the same mayonnaise flavor, much more will be necessary if spread on the top piece of bread than on the bottom piece of bread. In this manner, a more healthy sandwich can be built by minimizing the unhealthy ingredients and using more of the healthier ones.