When I first did colourwork, I learned to twist the yarns in the same direction at every colour change. This produces a very even result, but is quite time consuming, since you have to drop one yarn and pick up the other every time (…and I hate anything that interrupts the flow of my knitting).
A few years ago, I started holding both yarns on my left index finger, which is much easier and faster.
I like to use a ring to separate the yarns.
With this method, however, the colour to the left gets slightly longer stitches than the other, making it more prominent in the pattern.
So, you should carefully choose which colour you want to stand out more.
Stay tuned for more on stranded knitting and colour dominance.