I tend to use Google Docs both for jotting down ideas and for writing RP posts before posting them in PM or thread, because it's saved onto the cloud so it's saved every couple of lines and if there is a power or broadband cut I tend to only lose a tiny bit of what I've done as opposed to the whole thing (which is what happened before when I wrote things straight into the PM/thread XD)
Google Docs also has an app that I can download onto my phone that lets you work on stuff on the go and/or offline as well, to upload them back to the cloud later; so when stuff gets real quiet at work or while I'm waiting for buses on the way there/back I can work on posts and ideas and such as well.

I also like to keep a notepad and pen on my nightstand just in case I have a cracking dream and wake up in the middle of the night full of RP ideas I need to jot down XD