A soggy sandwich can mean a ruined lunch. A light spreading of olive oil on each bread slice gives you some extra flavour, and creates a barrier that keeps the moisture from taking over. Photo by The Marmot.
Gam Dias, the co-founder of MoData, shares his simple advice at Quora: spread a little oil on the inside of both slices of bread. You don't need too much, so just dip a clean knife in the bottle and spread evenly. For taste, you're probably better off with olive oil, but most oil-based spreads will work. Butter can work as well, but spreading butter on your sandwich may not be ideal for everyone. If you're sandwich ingredients are extra moist, however, you may be better off layering some plastic wrap when you pack your sandwich.