I like breaking my pastels into two pieces (or even three depending on the brand). For me there are a few advantages to breaking them. First and most importantly, it gives me more manageable pieces. When I use a pastel in its side, a whole stick can make a mark that's too large for the way I usually paint. Another nice thing about having two pieces is that it allows me put one half into my studio palette and the other into my plein air palette. Essentially doubling the amount of pastels I have. Finally, if I don't want to use the second piece in another palette, it can work as a reordering reminder. I keep the second half in its original box, when the first half gets worn down enough that I need the second piece, it's a nice, easy reminder to order a new stick if I like that color.
I always recommend breaking pastels in half to my students and I often get asked how to break them, so here's the way I do it.
1. If the pastel has a label, peel it off (if you want to remember the name or number of each color once the label is removed make a swatch chart- here's how). For stubborn labels that won't peel easily, slide a craft knife under the edge of the label to get a tear started.
2. Use a craft knife to make score mark all the way around the stick wherever you'd like the pastel to break.
3. Snap the stick, and it should break along the score line.
Note: This is the process I use for whole sticks, half-sticks are already half a pastel stick, so I leave them as they are.
This works for both round and rectangular pastels, and will usually result in two (or three if it's large pastel) nice pieces no matter the pastel's texture. There are a one or two brands that tend to break irregularly, but I've found that those pastels are often already broken before (or while) their wrappers are taken off.
If you have trouble getting the stick to break cleanly, try making the score mark a little deeper.