Opt for the Functional Horseshoe Design
Fire pits are traditionally rounded, but a horseshoe design actually is far more likely to provide an even burn of the firewood. If you use rocks you find by the river, try to have the bottom of the horseshoe be a particularly big size, while the others are the traditionally sized rocks. Keep the rocks roughly the same size to ensure that a grill will sit at even height over the fire.
Place tinder, such as dry pine needles, kindling, such as dry twigs, and also logs of wood in your fire pit. Build the fire in layers to avoid a partial combustion that leads to uneven heat and difficulty in cooking.
If the campsite you chose is part of a cluster of sites that is likely to be used in the future, keep your well designed fire pit intact. Conversely, if you are backcountry camping and do not anticipate returning to the venue any time soon---and it does not look like others preceded you---dismantle the fire pit and return the rocks to the places where you initially found them. Minimize the visible signs of the fire by packing out burned ashes.
Use Pre-existing Fire Pits
If there are already fire pits at your campsite, do not add to the environmental impact by placing another fire ring on the site. Double check the feasibility of the pits. Before you even begin using a pre-made fire pit, carefully consider its location. If the wind picks up, a fire may grow rather tall. Ensure that there are no tree branches hanging over the pit and that there are no trees within an 8-foot radius. If the pre-existing fire ring meets all of these safety specs, you are free to go ahead and use it.
Practice No-Trace Camping with a Camp Stove
It may be tempting to just go out, collect a few rocks, and then place them on the ground. Instead, consider the practice of no-trace camping, which is the most environmentally friendly way of providing cooking heat. Dig down about a foot and hollow out this area which will hold your camping stove. Once the indentation is made into the ground and the area is cleared of leaves and dry twigs, you are ready to place your camp stove into the ground. Opt for a lightweight MSR Dragonfly stove, which burns pretty much any kind of liquid fuel. Since the indentation in the soil serves as fire safety mechanism, there is no need to haul rocks from the nearby river, and thereby cause any form of environmental upset.