oooh, I don't think anyone is going to like my suggestion...but I'm going for it anyway 'cause I'm perverse like that!
In the beginning it's all about being gung ho and motivated and everything is all shiny and new and fun and woohoo the pounds are dropping and yada yada yada.....
At some point in time there will come days when it feels more like a chore, a burden, one more crappy thing you "gotta" fit into a busy day. A little insidious voice whispers "hey, I really don't have to do it today if I don't want to" and motivation is hiding under the bed.
And when that happens, just IMO, the only thing you can do is make like an old Nike commercial and "just do it" anyway.
You can change your routine frequently to keep getting results and keep it fresh, you can have a reliable work-out partner, and you can have a back-up work-out in place as an occassional "fun" replacement...I do all of those too, but in the end, I think the most important thing is to start building the self-discipline in the very beginning...you know - just showing up prepared to get it done day after day, accepting that some days will be golden and others dog poo.