This band-aid solution can give you a bit more time to replace your stiff shift or throttle cables.
When your shift or throttle cables get stiff, replacing them is always the preferred solution but unfortunately control cables are expensive and may represent an unwelcome stress on the household budget. Here's a temporary alternative that costs almost nothing and can keep you out on the water for an additional season or longer. The idea here is to lubricate the cable inside the jacket. Lubricants applied to the ends of the cable tend to penetrate only a short distance, often failing to deliver any meaningful benefit.
To lubricate the entire cable, you must remove it from the boat. This can be dead simple or an all-day job, depending on how the cables are installed and routed on your boat. Control cables that are really difficult to remove and install make a strong case for replacement rather than the band-aid repair of lubrication, but if your cables are accessible, lubricating can be an expenditure-deferring alternative.
With the cable out of the boat, insert one end (jacket and all) through a hole you make in a bottom corner of a heavy-duty zip-seal bag. Gather the bag around the jacket and tape it tightly with duct tape to seal the bag to the jacket. Reinforce the diagonal top corner of the bag with the duct tape. Poke a hole through the center of the reinforced area and use it to hang the bag so that the cable hangs down vertically its full length. Pour enough motor oil into the bag to fully submerge the end of the cable jacket and zip the bag closed.
Place a container beneath the bottom end of the cable to catch the oil that should eventually drip out of the lower end of the jacket. Allow the cable to hang until the oil drains through. Slide and rotate the cable in the jacket, then give the bag a second shot of oil (it can be the same oil) to assure a thorough lubrication. Remove the bag, reinstall the control cable, and you should be good to go.