All of the above advice sounds solid.
I'd also recommend being confident in what your fishing. People seem to perform better and make better presentations when they are fishing bugs they are confident in.
I'd also recommend trying to always make great presentations. If your presentation is right, and your not getting bites....you know your pattern is wrong.
Wild mountain trout can be a lot less picky, but if there is a hatch going on.....you need to be able to pick up on it. You also need to identify quickly what stage of said bug the fish are taking.
Now this is where you need to work on how to match the hatch. I'd shoot for matching the size and fly type first over color most of the time. Then try to match color if you've still not busted it.
Same case for tailwaters. You just have to be ready to go smaller.
Now, in times of no hatch, and no fish activity, pattern can be crucial. But the best pattern in the world will only be effective as the presentation that put it in front of the fish.
Anyway, Lots of good advice already posted.