What do you mean, "prune"? The only thing to do with a sago palm is do take off fronds that have gone yellow or have frost damage. Make sure to use palm food fertilizer to rejuvenate them, or mix 1 cup epsom salts in 5 gals. water. They only need this about every six weeks during the growing season.
Thank you for your response. In Northwest Fl. the fronds of Sago Palms are usually completely removed in the fall and left as is until the new fronds appear in the spring. About three years ago I had done this but now my Sago is overgrown. Since I hadn't removed the fronds in the fall I wasn't sure if it was too late to remove them now. I did proceed and removed the old growth leaving the newest set of fronds. I have many pups underneath so now I need to remove them to plant somewhere else. I attended a garden tour of five gardens today and saw some beautiful Sago palms. I saw many male specimens and two females with the seeds in the nest. They were beautiful !
If you are referring to Cycas revoluta, it is a Cycad, not a Palm.
Do not remove any fronds from the Cycad unless they are dead. As the fronds are dying back, they reabsorb nutrients, so removing the old fronds can stunt the plant's growth if they aren't dead yet.
In general, when a Cycad is flushing with new growth, it will be dropping the old leaves. When they are brown, it is okay to remove them.
Make sure you fertilize the Cycad when it is flushing with new growth.
I also wouldn't prune a sago palm. How big they get seems to be directly related to how large the pot is if they are in planters. Here in Gainesville we have some really huge ones, about six feet tall, growing in the ground. A lot of people use them as driveway borders, one on each side of the driveway.
I too, would never prune a palm, except to cut off entirely dead fronds. They do not like to be disturbed and I have also found that a pruning wound leaves them susceptable to infections. Also. I would NEVER use palm fertilizer. 1 teaspoon of epson salts in a gallon of water seems to be all they need, especially helpful if the fronds begin to yellow.
Basically, treat it as a native and ignore it, except to enjoy it.