I saw Ms. Socks today, and she asked me about lifelines in lace knitting.
When you know your stitches are correct, like before you start a pattern repeat or after a repeat, put your knitting down. Grab a tapestry needle, and some number 10 crochet cotton. Cut a piece of cotton about 2x as long as your knitting is, and thread the tapestry needle with it.
Put the tapestry needle right next to your knitting needle. Run it through the loops on your knitting needle, right next to the knitting needle, so that the crochet cotton is going through the loops parallel to the knitting needle. This is exactly the same motion as when you are putting stitches on yarn in order to hold them.
Once this is done, ignore the crochet cotton. Just keep on knitting. You will always be able to rip back to the lifeline
Link to someone else's video:
http://www.knittinghelp.com/knitting/videos//bas-tech-more/lifeline-rds.mpg