This is adapted from my experience pouring beers at the bar. One thing I learnt was that the bigger the distance between the cup and the spout, the more agitated the drink gets and hence, the more foam gets created. This helps in created a "head" for beers but in the case of normal soft drinks, it can be used to pour something without much foam.
All you have to do is tilt the cup around 45 degrees and pour so that the soft drink cascades down the side of the cup. If, for some reason you wanted some foam for say, a beer, slowly tilt the cup upright once you've reached halfway.
Enjoy!