For instance, recently my character has been planning the building a fortress. We've been in the area that the fortress will be built upon, and while there, my character has been scouting locations for the fort. Between sessions, I've been submitting thoughts and sketches for my GM to look over, and it promises to be a big story element later on. In games past, when the characters have pursued out-of-session-but-in-game (does that make sense?) pursuits, the game got a lot richer for it and the characters developed stronger personalities and motivations from that.
While out-of-game work can be advantageous, there's an upper limit. The character who is always designing a new starship, a new discipline, a new edge, or a new +6 Sword of Plant Slaying (Ginsu knife of sharpness!) is rarely contributing to the game, and is taking up a lot of the GM and other players' real-life itme. It's a balance. I like to see it, but often, players don't think outside the session when it comes to what drives and motivates them. While they often generate backstories at creation, they seldom remember to drive that story further in the context of the game. This usually has to happen a little outside the session, unless the rest of the players want to be bored to tears by his love interests or miscellaneous other character developments. (yes, I know, some of you play vampire and Loooooove that stuff. Just kidding.)
The big element for me is this: What' the best way to represent this between-session stuff? So far, I've done it via e-mail. I think this works best for our schedules. Do any of you have methods by which you handle between-session activity? Are there any game systems I've not had exposure to that handle this kind of thing in a balanced way? Enlighten me.