Lifelines can also be added as you knit, especially easy with interchangeable needles that have a hole near the needle and cable join large enough to insert the end of a crochet thread (contrasting color).
Pass enough thread through the hole to allow you to secure it as you knit in your pattern across a specific row. Do NOT cut the thread until your entire row is knit, and only cut it when you've stretched out the row and have enough to hold the stitches and another five inches on each end. Take the thread out of the hole (only the hole) after you've knit the row. The thread should be through all the stitches on the row.
When knitting the next round, just make sure you do NOT knit the thread into your project.
Should you need to frog a section of your knitting, the crochet thread will hold your stitches at the row where you added it.
If your project is extensive try to add a lifeline after every repeat.