Probably the best option is any of the many, many commercial lubricants designed specifically for sexual activity. Right now I've got a bottle of water based lubricant (I prefer "ID" or "Wet" brands) tucked away in a dresser drawer, and it's practically my best friend. Not only is the level of lubrication good, but it lasts for a long time and is easy to clean off since it's water-based. Do be sure to get one that's water-based, which will wash right off. Anything that's oil based is harder to get clean, and can lead to things like bacteria growing on it. Some of the lubricants have color or flavor, which in my experience make them stickier and less good, but others add sensitivity-enhancing chemicals, some of which can be quite enjoyable. The ID that I have definitely makes things a touch more sensitive during masturbation, and leaves me with a hint of a cool, menthol-like feeling afterwards.