I discovered the "life" saving value of a lifeline the time I was knitting the Estonian Lullaby Baby Blanket (pattern here: http://www.fibertrends.com/product/149447/CH41/_/CH41_Estonian_Lullaby_Baby_Blanket). It's a beautiful pattern, but I was fairly new to knitting, so I was just mortified when I had to rip back several rows because I had made a major mistake further down the blanket. It was even MORE mortifying when I had to then put all those stitches back onto the needle. It wasn't very long after this that my yearly eye exam revealed that I needed reading glasses. Coincidence?
Anyway, a friend suggested that I use a lifeline for the rest of the project. For those who do not know, a lifeline in knitting is a long piece of yarn that you thread through the live stitches on your needle for a row. It doesn't affect the gauge (I usually use a much thinner piece of yarn than the working yarn). The beautiful thing is that if you make a mistake, you can just rip out the knitting down to that row, and then easily thread those stitches off the lifeline back onto your needle. After you've completed your project, you can then gently pull the lifeline out. No more picking through the knitting, using a crochet hook to pick up all the stitches that you're trying to get back onto the needle, because of course they've dropped--cursing and swearing the entire time.
People usually associate a lifeline with lace, but you can use it for any complicated knitting pattern. In choosing a row for your lifeline, it's best to pick one from which it's easy to start over, say Row 1 of an 18-row repeating pattern. For instance, if you have to rip back any cable pattern, it's almost impossible to figure out how far down "Row 1" is, I don't care how easy the repeat. Ditto for any "holey" pattern or any complicated repeating pattern.