I know for a lot of us this will be common knowledge and, therefore, redundant. I just wanted to mention for the sake of the newbs that lather has three parts; water, soap/cream (aka product) and air.
I see a lot of threads that are properly ended with suggestions such as, "use more product" or "use more water." However, I rarely see the phrase "use more air!" That is probably because saying to use more air sounds stupid but I have found that it is actually sound advice in some cases. There is a reason that when you watch the shave videos on youtube (such as Mantic's) you see the shavers working the lather on their faces with their awesome brushes for a considerable amount of time. Aeration can completely change the cushion, slickness and consistency of a lather whether it is accomplished through face or bowl lathering.
My example from today: I was using cella and my Simpson duke 2 to prepare a lather for my straight razor shave. I like to load a lot of product with cella but basically use Marco's method with a slightly drier brush. After loading the brush, I looked at it and noticed that the lather looked thin and runny. It also contained many larger, dish-soap-like bubbles that are not good for a shaving lather.
As I stared at my shaving brush (and my fiance walked by shaking her head at her lunatic future husband), I remembered the first time I shaved with cella and painted that thin bubbly mess onto my face only to have it drip off and leave a sweet smelling but useless film where I expected the often celebrated Cella goodness.
So, I am writing this post for the newbs out there who are wondering why the soaps everyone else rave about are not working for them. Sure it might just take more water or more product but me... I used more air and it worked!