A few points for advanced imagers:
1. Flats should be about 66 to 75% full well capacity (ADU). You need to be sure you know your ADU (see
www.irishastronomy.org/index.php?option=...9&id=87780&Itemid=40
)
2. Flat signal level shouldn't vary by more than 1 or 2% between frames
3. Sky flats are probably best in terms of light source and uniformity - twilight T-Shirt flats worked well for me (but see 4 and 5)
4. Light-boxes are far more convenient (don't need a clear sky) but the illumination source needs to be chosen carefully and it needs to be as uniform across the field as possible
5. For narrowband imaging, daytime sky flats are probably best (unless your light box is very powerful or you are happy to take long duration flats)
6. Don't forget to take flat-darks
  