My father taught me a great way to carry a rifle in the woods. Oddly, it doesn't seem to be very common, and really should be taught and utilized much more than it is. It's very simple - and very effective.
All you do is place your off-side arm (the left arm for right-handed shooters) through the sling, and allow the sling to draw tight against the back side of your upper arm. Some adjustment of the sling length will be necessary to get it just right, and you may need to adjust that length depending on the clothes you're wearing. In the end, your forearm (the one growing off you, not the one on your rifle) should be roughly at right angles to the gun.
On the photo, my hand is open to show that the tension between sling, arm, and gun is what keeps the gun in place. Grasping the gun certainly helps with control and should usually be done, but it's not necessary at all times.
The rifle in the photo above is not light (it weighs nine pounds without any ammo), but I can still carry and control it this way with just one arm. I can even shoulder the rifle without using my strong hand, which illustrates just how much control this method provides. With a short, light gun such as the Ruger 44 carbine on which I cut my deer-hunting teeth, control is nothing short of exquisite and ease of carrying is greatly improved over conventional sling use.