I use the marking pens made by Dritz. Sold at WalMart (and many other places, I'm sure.) I've tried others but wasn't particularly impressed.
The water soluble pen marks disappear almost immediately after getting a good spritz of water - even after I've ironed over them. (BUT the instructions for both pens say to NOT iron over them, so I'm not recommending you do that unless you're as willing to gamble on such things as I am.)
I live in Coastal Georgia, where the air is almost always at least a little humid. I've found the air soluble marks disappear within one hour if I use a light enough hand. They've never lasted more than a few hours - and never as long as 24 to 72 - even if I iron over them. One good spritz of water and they're definitely gone, gone, gone.
The Dritz web site offers a few hints if you're having problems getting your marks to go away.
http://www.dritz.com/askus/faq/faq_05.php
Don't despair, gjs. If you haven't finished the quilt yet, just keep going. When you're done, I wouldn't be surprised if one good frolic in the washing machine with any mild detergent solves the problem.
If for some reason you can't get them out no matter what you try, then get some paint pens and turn those "mistakes" into artistic embellishments. Nobody but you need ever know the truth. ;-)