You can and should divide rhubarb plants every four or five years to give the plants more room and reinvigorate them. The best time to do this work is when the plant is dormant, so November to Feburary will work.
Dig the rhubarb using a fork to lever the crown carefully from the soil, trying to avoid as much as possible damage to the roots. Once you have it out of the ground, remove any rotting sections from the plant and give it a general clean-up.
Then, using a sharp spade, press down heavily and divide the crown into three. Ideally you want each section to have a crown, a large amount of root and at least four or five pink buds.
Then dig a hole for the divided crowns and place one crown in each, roots downwards, with the top of the crown roughly an inch below the soil surface. You can pop some well rotted manure or compost in to the hole first if you want to add some nutrients. Backfill with soil, firming in gently. New plants should not be harvested too heavily in the first year.
Of course, if you don't want more rhubarb plants, you can bring the new plants along to your local GIY meeting and share the largesse with other GIYers!