Back in the day, boot soles were stitched to their leather uppers with a sturdy, ropelike thread. Nowadays most boots are glued, and sometimes the glue loses its gumption. If your sole starts flapping in the field, duct-tape the bejesus out of it and pray that it holds until you get home. Then get to work on this easy and more permanent repair job.
1 Clean the boot with soap and water and let it dry completely. With rubbing alcohol, wipe the parts of the sole and the bottom of the boot that were once connected.
2 Stuff the boot with newspaper.
3 Smear the sole top and boot bottom with a contact cement designed for shoe repair (like Shoe Goo or Barge). Let the cement dry until it is tacky.
4 Reunite the sole and upper with gentle pressure and a steady, accurate hand. Once the two parts touch, you can move them around a bit for a better alignment, but it'll be tough to pull them back apart.
5 Use a C-clamp or duct tape to secure the boot sole in place while the glue cures overnight.
6 With a first-aid syringe (available at most pharmacies), inject seam sealer into the groove to prevent water and dirt from seeping between sole and boot.