I found wireframing a quite interesting technique when planning or updating my UI.
There are many tools available, online or offline, free or paid, collaborative or alone, let alone different functions included.
One I found available online and with no prior sign-up required is https://wireframe.cc/
Did you know UI wireframing? Feel free to share your thoughts and suggestions as well..
GUI wireframing
Re: GUI wireframing
Not something I use often, but I believe you've made some wireframes for Mudlet already?