How do you write effective Prometheus alerting rules?

Effective Prometheus alerts follow these principles:

groups:
- name: api-alerts
  rules:
  - alert: HighErrorRate
    expr: |
      rate(http_requests_total{status=~"5.."}[5m])
      / rate(http_requests_total[5m]) > 0.05
    for: 5m  # Must be true for 5 minutes before firing
    labels:
      severity: critical
    annotations:
      summary: "High error rate on {{ $labels.service }}"
      description: "Error rate is {{ $value | humanizePercentage }}"
      runbook: "https://wiki.internal/runbooks/high-error-rate"

Key practices: Use for to avoid alerting on momentary spikes. Always include a runbook link. Use human-readable messages with $labels and $value.