What are you using for string? Sometimes (often...) the cords will be unbalanced going in, which will tend to show up as bias in the finished braid. The good news is that braids are one of the better ways to take care of this - the crossings of the cords in different directions balance each other out.

Make sure that when you are braiding you aren't putting more tension on one side or another. Quite often the problem is that you left hand isn't as strong, so you end up pulling a little harder. Aim for consistency (I get the best results though when I don't think too hard) and give the completed braid a tug every now and then as you work.

Three strand braids usually work themselves flat pretty well. The round braids on the other hand almost always spiral for me unless I pay attention.

Matthias

Why remove it? Oh well, but info like this is goof to keep around for the next time someone has the same question!

Matthias

Also, try braiding using a flat surface to work it on or keep the strings and the braid tight as you're working it, and it should keep twisting to a minimum. I took a flat piece of wood, hammered a nail into it at a shallow angle and made a braiding jig. So, when I make a sling, I make the finger loop, and slip that over the angled nail, hold it down with my feet pull the strings towards me, which keeps them under tension and makes a flat braid. If you imagine that the *** is the board, and the \ is the nail, the whole thing looks sort of like this. The nail points away from me to anchor the string and keep it from moving towards me.
        \                                           (me---->)
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              (about 18" long/46 cm)

I safety pin my string to another surface, like my bed.
If its thicker string, I use a kilt pin.
You can pull it as tight as you like without curling it or twisting it.