I suppose by annual geranium you mean pelargonium?

The answer is (almost) easy: Except for some botanical pelargoniums like P. x 'Ardens' and P. sidoides you can't divede the plant. What you do is to take cuttings. An d the pelargonium loves it :) It makes it more bushy.
The cuttings may be everything for milimeters to 10 cm, but somethhing in between gives you best result.
Do you live where there is spring time now? Thats the best time to do it.

Good luck :D

Pbaby, let me tell ya how to go about getting plants from your cuttings, I did mine about a month ago, its not too late for you to get bushy healthy flowering plants for summer. Take your cutting about 5 inches long, cut the piece off the plant just above a leaf. Now cut them stem of your cutting up to just below the bottom leaf. Now pick off the bottom leaves and leaflets beside the leaf stem. Leaving just a few leaves on top of the cutting (not too many due to transpiration the plant will loose more moisture with more leaves) dip your cutting into rooting hormone #1 (a few bucks at any garden centre). Get a couple of the leaf nodes covered in hormone powder and shake off the excess powder. With the pot prepared with a light soilless potting mix, use a sterile object (pencil or popcycle stick) to make a hole. Bury the stem about 1 1/2 - 2" below the soil, and lightly pack the soil around the stem part that is below soil level. Make sure your soil is previously moisten before planting. Do not cover like other cuttings, these do not respond well to high humidity and are suseptible to root rot. Mine barely even wilted are apparently must have rooted fairly quickly as per lack of wilt. I have so many geraniums now, with all the ones I seeded and my "babies" from the two mother plants, I can't wait to see the show. It really pays to keep a couple of these over winter, now momma plants are ready to flower and I could get even more cuttings, but I've run out of space. Enjoy your Pelargoniums, Janet

I too keep my Pelargoniums in my basement under grow lights throught the winter.They look awesome right now.I have never tried taking cutting from them.But I can't wait to try it now.Should I put the cuttings in a sunny south window, or keep them in the basement under my grow lights until it warms up.Right now there is 2 inches of snow on the ground :-(

Another way you can use for zonal pelargoniums is to snap off a branch about three inches down from the tip. Use new wood.

Take off the bottom leaves, if any - and leave the 'cutting' on a bench to dry/callus over on the broken end. This takes one or two days.
Don't worry if the leaves wilt a little.

Take a three inch diameter plastic pot that's a little deeper than your cutting and fill to nearly the top with sharp grit no bigger than an eighth across. Ordinary builder's sand is useless because it holds too much water.

Push in your cutting with the callused end so the gravel lightly scores the stem.

Dip the pot in water and then let it drain before you put the pot and cutting into a cool place with bright but indirect light. If the cutting is getting reliable rain don't bother watering unless the weather is hot and dry. Otherwise keep it lightly damp but not wet.

Most cuttings taken this way will root and grow on. Duds are easy to spot because the stem blackens. Remove them promptly if you have done a number of cuttings and they are in a pot together.

You can also do this in the early fall and then over-winter your new plants.

These plants can tolerate some frost. If it is five degrees of frost or less the stems blacken and go to mush in the usual way - but you can get regeneration from the base. The plant is not as vigorous, however, but you may be able to save a favourite colour.

Another way to keep your stock of plants up is to plant out the seeds which is always an adventure.

If you are carrying your plants over from year to year they can get to be both leggy and woody - and taking stem cuttings is a good way to 'keep' the plant - without the aged appearance.

Thank you for your responses -- you actually just brought up another question -- my geraniums indeed have gotten very woody, and (yes, excellent I will take clipplings) -- is it OK to cut back into the woody part pretty low? Will new shoots grow from that or am I stuck with six inches of wood at the bottom! Thanks to all for detailed instructions, btw!