Sketches, sketches and more sketches. I have tons of books full of sketches, concepts, iterations and notes. I pride myself on my ability to communicate with team members and clients in a visual form. Collaborating and fleshing things out always result in a stronger solution.
Each project I work on gets a book... each book gets filled up! These quick ideas eventually develop into killer wireframes, storyboards, workflows... and more!
I also spend time figuring out ideas on the white board. Whether alone, working out some kinks, or in a team setting the early stages of brainstorming concepts, page layouts, user flows, etc in a "down and dirty" fashion is always a great way to go.
Although "down and dirty", the ability to see an idea and quickly iterate it, helps me to ultimately communicate and reach a better solution faster on every project.
Economic user testing! Especially for mobile products, I use the "POPapp". By quickly drawing sketches of each screen and setting up the flow, it works like the real thing. Another quick way to get something built and learn what works and what doesn't. Great for fast know on how to validate or iterate.
Storyboards often help me effectively and efficiently communicate with a team or client. This is a quick (hand drawn) "icon based" story which illustrates the critical path a user would take from one stage of the product to another.
Storyboard of a short animation, which ran on a website home page, was first worked out in my sketchbook.
Comparison of the original whiteboard drawing from a collaborative brainstorming session with the cleaned up Adobe Illustrator version for stakeholder review and discussion.