"option shift -" like so —, or if you want an en-dash, just "option -" like –. The funny bit is that I like em-dashes too as they indicate a pause or an interruption in thinking that just looks better than parenthesis, however I sometimes like to use en-dashes as well, which can be a pain since some fonts, especially monospaced ones, render exactly the same, for example in this HN edit box: - – —