I usually slip the first stitch of every row when I knit shawls, and you can see how I do it here. The main rule to remember when slipping stitches is if you're working in garter stitch, slip the first stitch purlwise with the yarn in front, then take the yarn between the needles to the back if you're knitting the next stitch (or doing Norwegian purl).
I always do two or three stitches in garter stitch at the edges in my shawls and scarves. So the first and last two or three stitches are in garter stitch.
When I started designing stripey shawls, I was looking for a way to join in a new colour that looked as neat as slipping the first stitch of each row.
Fenella is the shawl I use to show how this technique looks in the video.
Here's how I join in a new colour. In this example I'm changing colours every right side row. The wrong side row before I change colour, I knit/purl to the last stitch, slip the last stitch purlwise with the yarn in front. Turn so you're ready to work the right side row. On the right side row, just knit the first stitch with the new colour. You may wish to leave the first stitch slightly looser so that it'll stretch as much as your fabric needs to. Just give your fabric a stretch then carry on working the right side row.
When I was filming the video, the postman arrived halfway through filming so when I start demonstrating the actual technique, it was a separate video and the transition isn't that smooth.
You can also watch this video here.