Probably the most frustrating thing about lace knitting is the big mess you can make of things when you make a mistake and have to rip back in your work. If you made a mistake on the row you are knitting or even the previous row, unknitting or tinking is the way to go, but what if you need to frog a couple of rows?
All those yarn overs and decreases that make lace knitting so beautiful also make it difficult to properly reassemble a row when you've taken the stitches off the needle to rip out rows. So what's a knitter to do to save her work and her sanity? Use a lifeline.
A lifeline is simply a piece of smooth contrasting yarn placed through the stitches on a row you know you've knit correctly. Should you need to rip out in the future, you can rip back to that spot, the lifeline will hold the stitches in their proper place and you can just put them back on the needle.
It's great to use a smooth cotton yarn for your lifeline. If you're making knit lace, that is, a pattern that has yarn overs every other row rather than every row, one of the rows that doesn't have yarn overs is the perfect choice for placing your lifeline. If every row has patterning, any row that you're sure you've knit correctly is a good candidate.
Thread your lifeline onto a yarn needle and just slide it through each stitch on the needle, running right below the needle. Make sure the lifeline is long enough to have a few inches of yarn hanging off each side of the work. You can also leave enough of a tail on each side so you can tie them together, ensuring the lifeline won't get pulled out before you want it to.
Once the lifeline is in place, knit on as normal. You can add a new lifeline every couple of mistake-free inches, either leaving the old one in place or taking it out and moving it up.
Should you need to use the lifeline, remove the stitches from the knitting needle and rip out the rows you've knit since the lifeline was installed. You won't be able to rip further; the lifeline catches the stitches.
Then just slide your knitting needle into the stitches in order (making sure you start at the end where the working yarn is not) and proceed with the pattern. You may want to keep that lifeline in place for a few rows to make sure you don't make the same mistake, but be sure you don't knit it along with the stitches on that first row.