I've actually had success fishing into large groups of trout like the ones your speaking of (thats where I cought the brown in my avatar). Everytime I've caught something in that situation its been either on a midge or micro-egg (both of which ranged from #20 - #24). I think the most difficult obstacle is finding out what they will eat in the first place. To do this, I'll go through every midge in my flybox, give it about 25 casts, and if nothing takes, move on to the next one. Once I find a midge that works, I use a tandem combo of that midge and a micro egg and can usually get one strike every 30 or so casts. Sometimes it's every 50 (sometimes never), the key is you have to have a lot of patience.
The second big obstacle is knowing when you have a strike. This is where the micro egg, and a good strike indicator come into play. Typically, these big groups of trout are in very slow, and very clear water. An egg can not only serve as a second strike indicator, but you can also see the trout take it. From my expirience, they will only take something if it floats directly infront of their face......about 10 times in a row, and it remains in their mouth for less than 1/2 a second till they spit it out. It's almost like they're saying "ehh, why not...". That being said, the trout get a long time to look at the midge/egg, so I use 7X.
If they take the midge, and you still have your eye on the egg, if you see it twitch, you got about 1/4 of a second to tighten the line, or the trout will have spit out your tiny fly.
Occasionally, you loose sight of the egg, and in that case, I have a foam wire strike indicator that I can wrap around my line. This indicator not only dips like other ones, but you can also see sudden changes in line direction which you would not otherwise be able to see using a bouey or other sort of floating indicator. Any time I see either the egg or the indicator twitch the slightest bit, I pull up, and 99.99999% of the time, it ends up being nothing, haha
I wouldn't call this a very efficient method of fishing, but you can catch them. If I spend the whole day doing this, I average about 2-3.......it requires very good reaction time and heaps of patience.