This article is from the Hiking/Backpacking With Canines, by Terri Watson with numerous contributions by others.
You need a collar with identification and a leash. I use a rolled
leather buckle collar onto which I attach the rabies, dog license, and
identification tags. My leash of choice is a 6 foot long, round
synthetic leash, with a loop for the hand and a sturdy snap. The leash
will often end up getting wet - I've found that the synthetics dry
more quickly than leather. Be certain before setting out that the
leash, snap, collar and buckle are in good condition and will not
break if the dog suddenly lunges after the rabbit that's been teasing
him unmercifully from just ahead on the path. Some people like to use
retractable-type leashes to give their dogs more freedom. While these
are fine where allowed, I've found that they more easily tangle around
bushes or other people. In fact, my current companion bears the scars
from when Tika ran behind him more quickly than I could follow or
think to drop the leash. He ended up with rope burns behind one knee.
Be careful! Carrying an additional collar and leash is a good idea in
case of loss or breakage.