For me nothing really triggers it. I just really love to touch myself. I guess it's a kind of game for me. How fast can I get my sef off before someone comes home. Or how many times can I cum before I have had too much. I'm the same way with sex. I'm into public sex, the thought of getting coaght is very exciting.
But over all I just love to play with myself. I'm a very sexual being.
It can be almost anything, but fairly reliable triggers are waking up in the morning, reading about sex (even in "technical" terms or in passing mention), being alone in the house... reading the Eden forums. :-P
I'm never in the mood to jack off, actually. I enjoy making love more than doing anything on my own, because I get pleasure more from doing things to other people. I only do when I am either on webcam with my partner or on the phone because we're doing that together. But I get an erection from practically anything. I'm very infantile in that way. Whenever I'm just looking at her, I want to make love to her and I end up getting hard, either mentally or physically.
For me, no matter how great partner sex is, there are times when I just want to pleasure myself. Practically anything can trigger a desire to do it, from watching a sexy movie to an erotic novel, or like Liz, being alone in the house or reading all the Eden Forum posts! I have also done it as a means to getting to sleep faster.