There are a number of different ways in which markers can be used to help in knitting a garment.

• Markers - either rings or lengths of thread tied into a slip knot and slipped on the needle - can be used to mark the start of a round in circular knitting. They can also be used to mark the position of a decrease or increase. Safety pins are also good markers for positioning increases and decreases, particularly double increases or decreases along a sleeve seam which is being worked on a circular needle.
• Lengths of thread can be placed at each end of a row, for example the finishing row in a picot hem, to show the correct place to sew the hem to. If using an end of row marker, rather than tying a knot close to the knitting it's better to fold the length of yarn in half after you've slipped it into the knitting at the position you want it, and tie an overhand knot near the ends, leaving a loose loop. This can be removed easily after the marker is no longer needed.
• Lengths of thread tied into slip knots can be used to show the divisions between repeats in complex work, to allow you to check that the stitch counts are correct.

Lifelines are another way of marking your work. Their purpose is to provide an anchorage in a complex piece of knitting such as lace, so that if you have to unravel your work you just unravel back to the lifeline and then slip the stitches back onto the knitting needle. If you want to use a lifeline, take a length of fine smooth yarn (such as cotton) in a yarn needle and thread the yarn needle through the stitches on the knitting needle. The lifeline should contrast strongly in colour to the knitting and should be fine in comparison to the knitting yarn, so that there is no risk of distorting the size of the stitches on the knitting needle. People vary as to how often they use lifelines, but in complex lace work you will probably want to insert a lifeline every 10 rows or so. Once you have a lifeline in place the ones below it can be removed.

In addition, lengths of thread are useful for controlling groups of live stitches until they are needed. Sometimes using a traditional stitch holder causes the knitting to stretch out of shape and using a length of thread instead allows the stitches and knitting to maintain their position until you need to put them back onto the needle. This also helps to prevent holes in the knitting. It's useful for example if you are knitting a sweater from the neck down and need to put the sleeve stitches onto a stitch holder until you come to knitting them. Instead of using a metal holder, try the length of yarn instead.

I like to keep old balls of baby knitting yarn and knitting/crochet cotton to use as lifelines as I find that these are smooth firm yarns which do not fall out of the knitting too easily.