Do you write in your Bible? For me, it depends. I tend to leave most of them alone, saving the notes and highlighting for editions designed for it: wide margins. Here's a photo of my Cambridge KJV wide margin opened to Ephesians 1. The notes are for a class I taught on the text.
Over at The Foolish Galatian, Matt Blair has gone crazy with his Zebra highlighter, marking up his new Personal Size Reference ESV. As I mentioned yesterday, I've been reading mine daily and really loving it -- but I haven't worked up the nerve yet to write in it (and probably won't). Now I can experience it vicariously through Matt (and so can you).
Matt also links to an excellent post by Jesus Saenz on what kind of pen to use when marking your Bible (a Pigma Micron). In the past, I've used regular ballpoints, which don't seem to bleed through as much as my usual rollerballs. If I ever work up the nerve to mark up my Personal Size Reference ESV, I will now know the best tools for the job.