I do a lot of UI sketching. Here are my two tool sets:

If someone else is paying:

• Quattro Journal, Grid for straight lines and nice spacing (if you're sketching UIs I can't recommend this one enough)
• Prismacolor Premier Fineliners for outlines
• Copic Sketch Markers for shadows, highlights and where ever else needed (I primarily use the toner gray)

If I'm paying:

• Blank computer paper
• Black Bic ballpoint

A professor I had for a sketching class made us spend half the semester only using these two tools—you'd be amazed at how versatile they really are.