First, lets dispel a common myth: Rock fire rings do absolutely nothing to contain, corral, or control a fire.
That being said, a fire needs 3 things: air, fuel, and heat. An overabundance of one will create an uncontrollable fire. Thus, keep the following in mind:
- Consult the local fire conditions. Public lands agencies will rate the fire conditions. Heed their warnings.
- Don't build a fire in high winds. Sparks can be thrown hundreds of meters into dry tinder and start a fire
- Be aware of fuel nearby - anything organic that touches or is near your fire can combust. Tall grasses, shrubs, even roots that cross from your fire to a fuel source can create issues. Look for overhanging branches above your fire.
- Keep your fire small. There is no need for 10foot flames. Smaller fires have less likelihood of throwing sparks or torching nearby vegetation
- Burn nothing larger than the diameter of your wrist -- this allows for both a clean burn (no hot logs that you will be tempted to leave behind), and helps keep a fire low
- Break or cut wood so that it fits within your fire -- sticking a 20ft log in one end and burning it down is a bad idea
- Consider using a fire blanket under your fire to reduce the chance of igniting below ground roots (which also reduces fire scars)
- Consider alternatives: a small candle, a lamp. I've backpacked professionally for 15 years and haven't had a fire in the back-country in the last 14.
Addendum: Being "defensive" also means being prepared if all goes south in a hurry. Have water, or a bucket of sand/non-organic soil nearby to help stop spot fires if it escapes your ring. A proper tool (shovel) should be on hand if car camping.