If it comes off the largest chainring while pedalling it's worth a try to shift down again and try to get it on while still pedalling. Try a few times. With luck you may get it back on before you've rolled to a stop :-) Otherwise it's possible to grab a twig or something to lift the chain with to keep grease off your hands, but I usually just get dirty.
Winter riding is much less about badassery and much more about bundle-uppery. - malkin
1995 Kona Cinder Cone commuterFrankenbike/Selle Italia SLR Lady Gel Flow
2008 white Nakamura Summit Custom mtb/Terry Falcon X
2000 Schwinn Fastback Comp road bike/Specialized Jett