Depends on the type of burrito and ingredients. For breakfast burritos I put cream cheese in them, and then smear a bit on the inside seam and it works well to glue it shut. For a texmex style burrito you could use a bit of refried beans, or sour cream. If you're wearing your burrito you can just sprinkle a bit of shredded cheese of any kind in the seam and when the cheese melts it'll hold it closed.