Most ideas come at times and places when you least expect it, and when you don't always have a note book around. Evernote seems like a useful tool. Capture things on whatever device is convenient, but most importantly, archive it and make it searchable. This archiving is the most important feature I think. Finding notes, mobile phone images, yellow stickies, I lose most of them.
Maybe a special case of Fred Wilson's "watch later" concept: stumbling on things when you do not have time to deal with it, putting it away somewhere for later access.