I usually do some research when trying to develop new flies for specific insects.  Those flies almost always work.  The flies that I tie when I'm just messing around aren't quite so successful, though.  Some of them are just plain ugly.  I have a ton of mistakes stuck in a piece of foam on my desk.   

Dozens. Some I'll never figure out. I'm never done with the ones I claim to have finished. It's amusing to see something I gave someone two years ago as a "finished product" and it doesn't look as good as what I have now.

The more time at the vise the better tier you become. In the beginning it's tough to figure out exactly how much material you need on the hook. As a beginner you will most likely use too much material. Stick with basic patterns to start with as you build up your skills, Hares Ears, Pheasant Tails, Buggers, all easy for a starter. Dry flies, forget about it for a while. You tube has lots of videos that are helpful. Check this out too, it was very helpful when I got into it:

http://www.troutflies.com/tutorials/

I apologize in advance if I posted this in the wrong forum. I have to say I do not tie flies in an imitative manner and I am not a pro flytier. However my flies catch good fish. I find for Putah frequently there is so little material on the hook the greatest challenge is finding materials that show a good contrast between each other when wet. Even if your fly is only made up of 3 materials those materials (forme) work best when they contrast with each other rather than blending and looking like a wet turd.
Otter-I think there is genius in your approach. I find best success with my own patterns, but they are really not much diff than any of those. I think its funny too that sometimes showing them something different when they have been pressured hard in some holes can be a trigger. I remember about ten years ago fishing in the first hole below the bridge pre spawn but low flow and watching the 4-5 guys struggle to hook up with fish there. I had on a size 14 gold wire copper John I tied and I hooked up great for about forty minutes. I don't know why but I think it was so different from what those poor fish were used to seeingt hat it made a big difference.
JT- you have a piece of foam with your mistakes-for mine it would be the size of a nerf football. For me a great deal of the fun in flyfishing is trying new things-it can be surprising what works!
Tie the fly. Drift the fly. Catch the trout. Repeat.

Packrat, it looks as though you have made the assumption that I am a beginner. JMO, too easy? sure, but it took a lot of failures to achieve a modicum of success.
The most effective flies I have tied are suggestive rather than imitative. Would anyone disagree that a hare's ear or p.t. for example is suggestive? I think certain parts of a given bug's anatomy become triggers for trout to feed. You have to find the trigger qualities in your patterns you tie. I have found several triggers in my patterns for Putah. I don't extensively flyfish a large variety of waters and I would never be dumb enough to assume what works in Putah is universal. I do not claim to be an expert flytier and I will always say presentation is more important than pattern.
Tying flies is an integral part of the experience of flyfishing. I began at age 14 (I paid someone to teach me basics) and I must say it is satisfying to this day to be able to sit down before fishing...Tie a couple flies and go catch fish.
My post about mistakes/ineffective flies I imagine has to hold true for 98% of flytiers. I cannot imagine someone so in tune with all of it that every fly they tie is effective...The cool thing is the feeling you get when you capture lightning in a bottle...or in this case...on a hook.

Tie the fly. Drift the fly. Catch the trout. Repeat.