A broken build should be the exception, not the rule. Insist that developers who have broken the build stop all other work until they've fixed it. If the build is broken too often, it's hard to take seriously the job of not breaking the build. A light-hearted penalty can help to emphasize this priority. This developer then has to tape the sucker to his office door until he fixes the problem.
Some projects establish a penalty with more bite.
Microsoft developers on high-profile projects such as Windows NT, Windows 95, and Excel have taken to wearing beepers in the late stages of their projects. If they break the build, they get called in to fix it even if their defect is discovered at 3 a.m. Build and smoke even under pressure. When schedule pressure becomes intense, the work required to maintain the daily build can seem like extravagant overhead. The opposite is true.