Arranging a heat reflector behind a small fire can greatly increase the felt heat output by sending the warmth toward you instead of allowing it to be scattered. Logs, a board, rocks can be used close to the fire. You can further focus warmth by placing a reflector behind yourself. A tarp will work and it can pull double duty by acting as a windbreak, making you feel warmer by reducing heat loss. Mylar space blankets work really well as reflectors.

Also a small fire placed closer and using reflectors is more effective, and fuel efficient, than a large fire.

If your thinking of something less logistically involved one of the most efficient methods of warming someone I know of is to have them crouch or sit and place a large garbage bag with a head hole cut into it over them and placing a heat source, candle, candle lantern, oil or carbide lamp, under the edge of the bag. The heat rises into the bag where it collects and warms the person. You have to be careful not to set the bag, ground cloth, or any clothes present, on fire but it works very well.

By varying the flame height of the heat source, the amount of air you let in under the bottom of the bag, extending you legs out of the bag, and using your head to open or close the head hole it is possible to regulate the interior temperature. If the heat source is one that is one you can reliably regulate to a low setting and burns a long time it is possible to nap in such an arrangement.

In cold conditions it help to place some insulation, assuming you have anything dry, between your back and the bag. Also don't make the head hole too large. A good quality garbage bag will stretch around you head a bit without tearing. This technique, like so many other things, works best if you have tried it a few times before getting it to work in a critical situation.