Odds are most of your ideas are crap. I know mine are. I have notebooks, wiki pages, and whiteboard drawings describing everything from new Facebook applications to wireless control circuits to stories that I'd like to write. If you're anything like me, you get ideas that would take up 10 times the time you have available. At some point you have to decide which ones you're going to try.
While this will vary from person to person based on your individual priorities, etc, there are a few things everyone should do:
Capture the Idea – Just as I wrote earlier, write it down somewhere safe… a notebook, your whiteboard, etc. The old geek-legend about a napkin would work too, just put it somewhere you won't lose it.
Wait – Let the idea sit for a few days – weeks? – and take some time to think about other things. If you immediately start on an idea without considering it, you'll often waste time going down deadends, bite off more than you can chew, and discourage yourself to the point of inaction.
Compile your Notes – Now that you've had a few days to think about other things, take another look at the idea. Odds are you thought of literally dozens of related points, problems, and pieces. Fill in all of those points and group them if you can.
Consider it Again – Odds are a few of the things you came up with make the idea difficult or even impossible. Some of the pieces – probably the ones that popped out to you first – are going to be the easiest. While you can't immediately let the difficult/impossible ones completely discourage you, you shouldn't let the easy ones seduce you.
Tell Someone About It – Yes, I know your idea is so incredibly innovating and amazing that telling anyone will risk it. But in reality, it's probably not… Stop worrying about it and tell someone you trust. Buy them a cup of coffee or drop them an email and pitch them on it. See what they think, see what feedback they have, see what criticism they have, and take it all with a grain of salt.
Some of the feedback will be great, some will be terrible, but the point is that if you can get someone else to understand it and see the value of it, then maybe 10, 1000, or 100,000 other people will do the same. If you can't convince your friends, family, associates of the idea: it's time to re-evaluate. You might need to simplify your idea or your pitch and try again.
If you try this a couple times and few still see the value of your idea… move on.