Summarize:

A blanket folded into thirds can remain styled in-place regardless of how many people sit on the sofa. Hold the throw out in front of you lengthwise and fold both ends of the blanket into the middle. You can then drape the throw blanket as-is or fold it in half to avoid one end dragging on the ground. Depending on your decoration style, you can drape the blanket on the cushions, arms, or back. Some throws (especially cable-knit blankets) look better with a little "organized dishevelment." Fold your blanket in half and drape it directly onto the sofa arms, back, or cushions. You can even toss it onto the sofa for a graceful-yet-natural look. Fold your throw in half length and then half once more. From there, you can drape it directly onto the arm of your sofa. Or, drape it over one corner of the sofa and position a throw pillow over the part that is on the front of the sofa. Not all throws need to be folded. Lightweight blankets look great folded, but thick ones (like sheepskin or wool) naturally look snug when tossed onto the sofa. Your sofa will seem pleasant and inviting enough to nestle in.
Fold the throw into thirds if you value structure. Fold the throw in half for a rumpled look. Quarter your throw for casual styling. Position an unfolded blanket on the sofa to make it look comfy.