Once the fermentation is generating a thick head of krausen, I have found it helpful to leave the lid partially cracked, allowing an airspace for the large amounts of CO2 to vent. With the ferment in high krausen, the classic dense rocky heads will form. At this stage, trub will be scrubbed from the ferment, and rise to the surface, along with other solid matter that was carried over into the fermenter. The ability to skim the trub and yeast that rise to the top of the fermenter is one of the main advantages of open fermentation. Don't overdo it, but about once a day or every other day, depending on the rate of ferment, skim the top.
Many ale yeasts tend to flocculate at the top of the ferment as the ferment diminishes.
When choosing yeast to save, be sure to wait a few days into the ferment so that the trub is scrubbed away and the harvested yeast is clean. As the ferment dies down, keep the lid over the vessel. Another great plus of open fermenting is the ease of dry hopping. Allow at least 3 days time for the dry hopping to take affect. I would avoid leaving the beer in the primary for longer than 2 weeks, and aim for 10 days when dry hopping, and a mere 5 days otherwise.