It's best to have a professional do it.
I would try to vacuum it first to see if the particles will lift without actually applying anything to the painting. Also, "sweeping" with a soft toothbrush is worth a try. If it doesn't come clean, and it's a valuable painting (either monetarily, sentimentally, or historically), I'd stop there and get a professional. An art museum or dealer in your area could recommend someone.
If you decide to keep going yourself, you can try: http://www.ehow.com/how_6596276_clean-smoke_damaged-oil-painting.html. Between step 3 and 4, I would add testing the neutralizer in an inconspicuous place 24-48 hours before to make sure there's no adverse reaction.
NEVER use household cleaners. I'd totally recommend taking it too a professional if you'd be heartbroken if it gets damaged any further.
Answered Jun 05, 2011
Edited Jun 05, 2011