simms is absolutely right. If it isn't anything very special a VERY weak solution of amonia and cotton wool will lift a lot of general dirt like nicotine. Be careful not to get the canvas too wet. Or you can get a sort of magic putty from Lion (look for them on google under picture framing supplies) which lifts dirt off. I think you'll have to order their catalogue though - the web site doesn't seem to give half the stuff they actually do.