I've given up on 'I don't know'. I only say that when I'm at the end of the line and there's nowhere to go.
Instead, I usually use anything else that indicates that I'm going to make inquiries. "That's a good question. I'll have to look into that." comes out of my mouth a lot now. Sometimes without "That's a good queston." at the start.
I like to form a hypothesis then test. Does that make me stupid for jumping to a conclusion? I'd rather approach something with "I don't know, but I think maybe...", lets test and see.
Define usually. More often than not when I hear "I don't know" it is offered as an excuse/explanation for laziness and shirking responsibility.