The upshifts on my mtb rear derailleur have been extremely sloppy all week in the cold weather. Tonight's ride home was particularly frustrating as it was taking 3-5 turns of the crank for the shift to occur.

I figured the cable housings were gunked up, so while I was putting my slicks back on I decided to clean them out. The little red tube that comes with WD40 works well for purging the housings, then I purged the WD40 by spraying in some aerosol teflon lube. Everything is smooth again, but I'll know for sure tomorrow morning.