Two of the most valuable accessories available to the field accuracy nut are the sling (military type) and the bipod.
The technique in this article depends on both, but either can also be used separately to increase shooting stability.
Harris (harrisbipods.com) has long been considered one of if not the premier bipod manufacturer. Models range from small benchrest or prone models to those designed for shooting sitting, as described here. Prices range from about $80 up to well over $100, depending on model. Harris also sells attachment accessories that allow shooters to fit a bipod on almost any rifle.
Several other companies also make or import bipods, and they range from excellent to less than serviceable. Do your research and talk to fellow shooters (and for a wide range of opinions, post a query in the chat room at rifleshooter.com) to find one that will work for you and the type of hunting or shooting you do.
Today's hunters tend to purchase the popular, good-looking cobra style sling, which works well for carrying but provides little if any stability in shooting situations.
A classic military sling, available from some gun stores and most shooting supply catalogs is much more versatile. It's both a carrying strap and a shooting aid.
With a bit of study and practice, a shooter can significantly improve stability in field positions. One has only to look at the achievements of competitive shooters to recognize how effective a sling can be.
Material is often leather or nylon, and prices range from about $20 up to several times that.