If you have a nail to hammer in and the position is just out of reach, in an awkward place, or there simply isn't enough room to fit in both arms to hold and hammer at the same time, a simple stick pulled from a tree will help you.
Find a fairly straight stick and cut a notch in it at one end. Then push the nail into the notch. It will be gripped quite tightly, and with the stick you can maneuver the nail into place.
Your hammer's handle will extend the reach of your other hand, and you can hammer in the nail easily. Once the nail is in, pull back on the stick (the nail will slide through the notch) and hammer in the rest of the nail.
This tip can also save your fingers if you're prone to hammering them rather than the nail head, so you don't need to restrict it to out-of-reach places.