You have plants that are near harvest and want to keep one as a mother plant?

You're going to have to re-veg whichever plant you want to preserve. Harvest early leaving the lower branches, leaves, and buds intact and place the plant under 24/7 light. It is going to take a while, but in a few weeks you will have new growth that you can take clones from. If you wait too long, however, the plant will be too far gone to re-veg so pick the one you want to save and harvest it early.

I just took several cuttings off of a plant that I've been reveging (I didn't keep any clones because I wasn't all that impressed with the plant - when I finally realized how nice it was it was too late to take cuttings), it took about 5 weeks before I could take clones off of it.

Reveging is interesting, for the first week or two you will see no signs of new growth, although the calyxes on the lower buds that you have left will continue swelling - they reach a size far larger than they would on a normal mature bud. Eventually, single pointed leaves with little or no serration will begin sprouting out of these buds followed, eventually, by three point leaves - likely wrinkled and oddly shaped - and finally normal leaves.

Once you get clones off of the thing you can make one of these your new mother plant.