I find that with planning and writing both, a map helps me an enormous amount, but have always struggled to produce useful maps myself (having the drawing ability of a toddler). I’ve been playing around with this all day, mapping out my world, and have found it incredibly easy to use. It’s still in alpha phase, so it’s a bit rough, but nowhere near as much as you’d expect given ‘alpha phase’.
Anyway, enough gabbling, here’s the link: http://inkarnate.com/
If you sign up for it (it’s free), you should get an email in the next couple of hours to give you access to it.