I use rubber cement, then using a hot glue gun "weld" the parts together with a thin strip doing a curvey solid smooth motion over the seam. Has held up extremely well. I suggest a "cold" glue gun for this though, as my extreme hot temps have melted the bond on the rubber cement before and caused it to pull apart.
Simply put a very very thin layer of rubber cement on the 2 sides of parts to be joined (normally a tab and a flap) where they would join (depending on the fold, one under and one on top) and let them dry (blow for 5 seconds on them) then line up carefully and press together. Instant hold. Finish the part if it has multiple tabs/spots, then let it "cure" for about 1 minute before hot glue welding. Don't glob on the hot glue, just a thin half moon sway will work.