Accidents happen, and sometimes they happen to our copy of the Bible. There are some precautions we can take, but there also some things that may help if that disaster does happen.
Wallpaper cleaner.
I'm not too sure about this one, but apparently wallpaper cleaner (available from DIY stores) can clean unwanted marks from the pages. (I might prefer the odd mark rather than risking it)
Tightening Loose Covers.
Gently open the area between the cover and contents. Dip a knitting needle or stiff wire in glue to evenly coat it. Insert between the cover and spine. Do this to both covers, close the bible and let dry over night.
Repair a Rip.
If you do get a rip 3M "Magic Tape" might help. Be wary of other tapes because they often become brittle and brown.
Rebind.
Some book binders offer a repair or rebinding service (Eg 'Bible Restorations').
Use at home, not out & about/Buy a new one.
If it's too far gone, or too expensive to repair it may be a good idea to purchase a new Bible to carry around with you. Keeping your old one at home will keep it and any notes inside that little bit safer.