Use a good quality seed starting mixture. Water the soil carefully and let it drain. Then sow your seeds. Cover with plastic and keep humidity high and temperature consistant (70-75F).

Damping off in my experience happens to impatiens most often if you vary the temperatures, use high fertilizer in the growing media or keep the media too wet/saturated. It should be consistently moist - and never be allowed to dry out - but if you use too heavy a mixture of keep it too wet, germination is problem and damping off a bigger problem.

My big trouble is double flowered impatiens. I almost always end up with damping off.

Ok, I forgot:
- For types/varities which I have had problems with I do this:

I divide the seeds in 3 lots and start them in 3 seperate containers. This cuts down on "loss".