I do 90% of my songs just sitting in from of guitar pro, and here and there I will take my guitar.
I litterally don't know how to play my songs, I really feel like in the pants of a composer more than in those of a musician when I write music. I'll learn them before I record them.
It's definitely all about going out of your confort zone.
None of my songs (in my current project) stay in the same key. Tempo is changing often , signature too, and I try not using the basic harmonies too much, and try using open chords. I sort of "invent" chords just by taking my guitar, randomizing the position of my fingers, or by drawing it on the guitar pro guitarneck.
I also try finding rythmic ideas, bass lines, synth/orchestra harmonies, to approach it from a different perpective.
It has worked well for me.
Another awesome way to work , is to find your musical soulmate, and you just send to each other your song, one adding or changing parts after the other. The rule is that when you get stuck or short of idea, you send it to the other who will find some. I have a friend like that, we wrote a full melodeath album like that and some songs are really fun to play, it's just frozen cause of practival and distance reasons.
The best way for me to get out of a writer's block, is to listen to a shitload of new music. I always end finding inspiration somewhere.
Also, playing randomly and casually your instrument for pleasure, improvising, without intending a processus of creation, can surprise you.