A Cable Cast On is a good way to cast on stitches in the middle of your knitting.  Often a pattern asks you to be knitting along and then says, cast on some number of stitches. I almost always use a Cable Cast On in that situation. This example comes from our Baby Bloomers Project Journal.

First, turn the work so the wrong side (or inside) is facing you. Insert the right needle between the first and second stitch of the left needle. 

Knit a stitch onto the right needle, just like you normally would., except do not remove anything from the left needle.

Now there's a stitch on the right needle.

Without twisting it, place that stitch onto the left needle.

It looks like this:

Repeat the steps for however many stitches you're casting on. Insert the right needle between the first and second stitches of the left needle. (The first of those stitches is the one you just cast on.)

Here is what 10 stitches cast on look like (after turning the work so the right side is facing):

A couple of tips: 

Cast on loosely. It gets really difficult to deal with a Cable Cast On if the stitches are too tight.

Also, make sure you keep track of how many stitches you have cast on. It can be kind of tricky to determine which is the first cast on stitch!