Short answer: Use Mona Lisa Brush Shaper

Ridiculously long answer:

The following assumes you are using these for oil paint. Obviously, for acrylics, goauche, watercolor, etc., you would not use solvents as described below.

Signet brushes are made with ChungKing hog hair bristles. Hog hair reacts to water the same way coarse human hair does. It curls out of shape. Think of how your own hair looks if you get it wet and don't do anything with it. This is because hog hair bristles (and human hairs) are hollow. When filled with oil, turps, or other suitable solvents, they can be manipulated to a somewhat straighter shape, but when soaked in water and left to dry they will curl if not properly handled. This is because the natural substance in animal hair is the oil secreted into and around the follicle by the hog. Some "experts" say never to use water to clean them, but this is wrong. Soaking with mineral spirits or turps and leaving to dry will shorten the life of your hogs. The solvents will eventually destroy the hair fibers and dissolve the epoxy holding the hairs and ferrules in place.

Here's what I do and it works perfectly. When I'm done painting, I wipe excess paint off on a paper towel for all of the brushes I used. Then, I thoroughly rinse in odorless mineral spirits. (Turpenoid, white spirits, turpentine, etc. all will do this job, too.) OMS can be used over and over. I then wipe them again on a clean paper towel. This removes about 75% of the paint on the brushes. I immediately proceed to washing them. First I use Goop (the original formula), without water, in the palm of my hand and then worked into the bristles thoroughly with my fingers. It works best if you don't wet the brush first. Then rinse in warm water, not hot. You want to get it rinsed out well, and Goop is very soluble in water so it quickly rinses out. It gets squeaky clean, literally. Then, I finish off by using either Jack's studio soap which will impart a bit of oil back into the hair, or Master's brush cleaner. I then wipe the hairs dry with a clean towel being careful not to pull on the hairs, but squeeze them gently on the sides to remove most of the water.

Now, to finally answer your question, there are two things you can do to keep the hairs straight.

1. The easiest and probably best way is to use Mona Lisa Brush Shaper. This stuff is simply awesome! Dip your brush in, shape the hairs with your fingers and let dry. (I always hang them upside-down to dry.) This sizes your brush. You may recall when you purchased brushes, they were nicely-shaped but hard. That's what this is. If you have really bad brushes that refuse to behave, you can dip again and shape until they do what you want them to. They stay in place after about ten seconds and since they are plastered there, they are "trained" to keep that shape. When you go to use your brush, you dip in mineral spirits (if you're painting in oil, use water otherwise) and brush the bristles back and forth until the dry size is removed. It comes right out as a powder. The directions say you can also just brush the dry brush vigorously back and forth in your hand, but this leaves some of that powdery residue in the brush. Better to just rinse it out because you'll want to do that any way. You'll find the more you use this stuff, the more your brushes are "trained" to keep that shape. I've taken completely horrible, scraggly brushes that looked like they should have been tossed and made them look brand new with the cleaning and shaping technique I just described. I mean brand, spankin' new! :P

2. This is a little more labor intensive, but works almost as well. It has the advantage of not having any substance on the hairs which has to be rinsed, but only really works for flat brushes like flats, filberts, and brights. Wrap them in news paper or phone book paper. Lay the freshly washed brush on corner of paper and roll up nice and tight, folding the end over to make a nice wedge. Watch this video: http://www.youtube.com/watch?v=KMcQbuxVlwY

Until I found the brush shaper, I used to use a business card folded over and a clip to get a sharp edge on my flats, but they tended to spread out and I wasn't really thrilled with the results.