Why lube a bearing?
A lot of people think that lubricating a bearing will make it spin longer. In this case, however, lubing a yo-yo makes it respond better. Provided you use a viscous lube, it will shorten the sleep time a little and increase response. It provides friction in the bearing, which effectively slows it down and makes it more likely to "catch," bringing the yo-yo back up to your hand. This is often a necessity for two-handed looping yo-yos.

What is the best lube?
A lot of people use Vaseline Lip Therapy, however there is no best lube; different lubes serve different purposes. Pictured here are several different types of lube. For string trick yo-yos, you may want to use a less viscous lube, so as to not decrease the sleep time by too much. The outer two, Brain Lube and Super Lube oil, work well for this. The others pictured are, left to right, Super Lube grease, Vaseline Lip Therapy (the most popular), white lithium grease (in the syringe),and plain Vaseline (in this case, Chapstick brand). These tend to work better on looping yo-yos, as they decrease sleep time fairly significantly. Try different kinds for your purposes, you may find something that works better for you. You can get a free sample of Super Lube from www.super-lube.com.

How do I do it?
Applying lube is easy. The goal is to get some lube in between the bearing and the spacers. What works well is to take off the bearing, squeeze some lube onto the axle, then slide the bearing back on. Put some lube on the other side of the bearing, and screw the yo-yo together. If you used too much, and some squeezes out onto the outside of the bearing, wipe it off; don't get any on the string.