I've been working on various projects with a few friends and a phrase keeps popping up: "I can't get any good ideas." This, of course, is nonsense. The people I'm working with have great ideas on a daily basis, it's nailing down the one at hand that's difficult.
What makes it so difficult is the Idea Inferiority Complex. Believing that an idea is bad at first glance and tossing it aside. This is why so many people recommend dumping your brain out THEN deciding which ideas are worth a second look.
Even then it's possible that an great idea might not strike you as being all that good. Remember, JUST DO IT started out as a disposable tag line. It was only later that the brilliance of that idea was recognized and embraced. It takes a lot of training to spot an idea like that and champion it.
This is why juniors are not Creative Directors. Juniors are meant to sit there and work all day on developing new ideas. Your best idea might be buried down the page. If you work for a CD worth his salt he'll be able to pick this out. Slowly you learn what good ideas look like. You get more confident and stop doubting your ideas so much.
It takes time (often what feels like an eternity) but the easiest way is to stop thinking your ideas are terrible. Many are, no doubt. But focusing on the good ones instead of focusing on the bad is a good place to start.