I've had great success rejuvinating knives with DuPont Teflon Multi-Use Lube, which is available next to machine oil and other lubricants in a home improvement store like Lowe's or Home Depot, and at some large Wal-Marts.
The Teflon lube is in a liquid state in the bottle - it looks like milk. After you squirt it on metal or plastic, the fluid evaporates and leaves behind a powder residue. This powder can be rubbed off with a cloth, brush, or Q-tip, and as it flakes off, it takes grease and gunk with it. I've purchased a few knives on eBay that were "new in box" but had been in storage for so long that they had stiff actions due to the factory grease having become like tar. The DuPont Teflon lube, applied liberally in two or three applications, make the locks and all other moving parts smooth as silk.
NOTE - this lube is toxic if ingested, so after you clean up a knive with the lube, you'll need to wipe down the blade with soap or some other non-toxic cleaner just to remove the Teflon powder (the powder is water soluble, so the "final rinse" is a breeze).