The easiest way I can think of is hand drawing them with a tablet. It's fast, and with a bit of practice, easy. Plus you get good control of where they start and finish.
I might be a bit biased towards hand drawing, but i think almost everything looks better when you do it by hand, coastlines, rivers, lakes, height and so on. It's all about controlling the map.
Generating coastlines can be good sometimes, aspecially when you can adapt the actual use of the map to the map itself. If i make a map before writing a campaign about it, it doesn't matter.