Elaborate masking

so…say i had a REALLY elaborate floral design going on and i wanted it to “grow” in. I know how to mask around it, but the way i know how to do it is nearly frame by frame. Is there a better way to do this? for example, I wish i could make a line “grow” into any shape i wanted, but i REALLY don’t want to frame by frame animate this as it will take FOREVER. Any ideas/tuts?

Right now i’m animating a circle on a guide, and then deleting the frames behind the circle as to show the line underneath…to me that seems like it’s too much work for the result and i’ve seen some really elaborate mask jobs but have yet to find the “trick”