First try using a blower, like the Giottos Rocket Blower. If that doesn't work after a few tries, use a wet swab. It's not as daunting as it sounds - it's really quite easy. Come to my work sometime, and I'll show you. Swabs = $3.95 ea. 60ml bottle of cleaning fluid that'll last years = $10. I clean sensors all day, and I haven't fucked up....yet...
It's a long, detailed read:
http://www.beauphoto.com/cleaning/index.html