If you've ever had to frog back in a piece of lace knitting -- or if you're scared to try lace in case you make a mistake! -- lifelines can be your friend.
A lifeline is simply a piece of waste yarn or cord that you run through a row of live stitches. The lifeline then stays in your work for as long as you need it, providing you with a safety net. If you have to rip back, you'll only need to go as far as the lifeline. Since you know all the stitches were accounted for when you put the lifeline in, you can pick them up from the waste yarn and start knitting again from the point where you placed the lifeline.
How to Place a Lifeline
To run a lifeline, you have a couple of options. Some knitting needles have a small hole before the cable join. You can thread your lifeline yarn through this hole, and it will automatically be pulled through the stitches as you knit. If your needles don't have that hole, you can use a tapestry needle, threaded with your lifeline yarn, to bring the lifeline through each stitch on your knitting needles (but NOT the stitch markers -- see below!).
Once your lifeline is placed, you can cut the lifeline yarn, leaving a tail of several inches on each end.
When you're done with the lifeline, just pull one end to slide it out of your piece. You may choose to leave your lifeline(s) in until the piece is finished, or you may leapfrog them, pulling out the first one you placed once you've placed a second one and using that first piece of yarn to place the third lifeline when you get to it. Up to you.
Tips for Using Lifelines
First, choose a relatively thin yarn -- definitely not any thicker than your working yarn. Slippery is best, so that it's easy to pull out when you're done with it. (In other words, skip the mohair!) It will be easier to see your lifelines (especially if/when you end up picking up stitches from them) if you choose a lifeline yarn that contrasts well with your working yarn.
Make sure you DON'T catch any stitch markers you're using when you thread the lifeline! If you do, the markers will be trapped down at the level of the lifeline. The farther you work, the more they'll pull and scrunch up your work. If you do accidentally catch one, just replace it with a non-tethered marker for the rest of the piece. You can reclaim the original marker when you remove your lifeline later.
Note which row you're placing the lifeline in so that if you do need to rip back, you'll know where in the pattern to start knitting again. It often makes sense to place lifelines at logical points in the pattern: for instance, after X number of repeats, or when you switch from chart A to chart B.
I find it easier to place lifelines on purl/rest rows, if your pattern has them, as all the stitches are uniform. But you can place lifelines on any row or round (and as often as) you want!
Yes, it takes a little time, and you may never use it. But the first time you need to rip back, you'll be so glad you placed a lifeline!