Measuring is the way to go!
I measure 3 times, average it out, cut the borders to THAT measurement, apply the borders by pinning the center of border to center of top. Pin ends to ends. Ease the rest in - pinning as I go.
Sew. Press open.
Repeat steps.
If the description doesn't make sense....check out this youtube video
http://www.youtube.com/watch?v=OkQjYVAgUGo
As a longarm quilter, I can tell who's applied borders correctly and who just sewed on fabric and cut the ends off.
I can tell because the borders WAVE at me!
Nan