Remove the plug with pliers, but save it. Removing the tangent can be risky, especially if it's a humid day and the wood has swelled. It's easy to break a tangent, often splitting it lengthwise under the torsional stress. Try first with your fingers to twist the tangent out of its hole. Twist gently, pulling as you go, and work the tangent out of its hole in the key.
Graphite works well as a lubricant.
The idea is to apply some graphite to the side of the key where it touches the keybox, and to the slot that the key rides in. Put on a lot, as it certainly won't hurt, and will keep you from having to do at again for many years. Blow away any excess with a puff of breath. However, often just lubricating the keys is not enough. You'll have to remove some of the wood in either the key or the inside of the slot.