You can choose the direction of next segment to make specific angle between segments (0˚,30˚,45˚,90˚, etc.). To enable this mode press A again and see helper lines while moving mouse. To fix the direction and move mouse far from snapping line, please hold A key.
Right click on any way segment when snapping allows to use that segment as reference (for example, to draw segment that is parallel, perpendicular or 45 degrees-aligned to the reference segment). Instead of the right click you can also hold Ctrl while moving mouse over desired reference segment.