Damping off is common when humidity is high, especially if seedlings are too close together and airflow is restricted.
There are a few things that can be done to avoid this:
Avoid sowing too thickly, and thin seedlings to their recommended spacings soon after germination
If sowing under cover (e.g. in a greenhouse or cold frame) keep it well-ventilated to improve airflow
Never reuse pots (especially if seedlings grown in them have suffered from damping off) without thoroughly cleaning and drying them first
Avoid overwatering - the potting mix should be allowed to almost dry out completely before watering again
Sometimes rainwater collected in rain barrels can contain pathogens that cause damping off, so make sure that rain barrels are kept covered, and are cleaned at least once a year. It may be preferable to use mains water for seedlings if possible instead.