So many people find my blog looking for instructions on how to knit mitered squares (due to my posting so frequently about my rug) so I thought I would try to help them out by posting what I know about it.
My first attempts at mitered squares were for the Mitered-Square Rug pattern from Interweave Knits Summer 2004 issue (available free online, see link below). I've got various photos of progress on that pattern in the UFOlympics category of this blog.
For that rug pattern you cast on 41 stitches and then knit one row. The next row you knit 19, slip the next stitch over to the right needle by putting your needle into the loop as if you were going to knit it (but don't throw the yarn to make a stitch) then knit the next two stitches together (K2tog) and then pass the the slipped stitch over the last stitch (PSSO) finish knitting to the end of the row and then knit the next row. So, every other row you do a Slip 1 knitwise and K2tog then PSSO in the middle alternating with a plain knit row to make a mitered garter stitch square. So it goes K19 on the first row until you do the decrease, K18 on the third row, K17 on the fifth row and so on until you get to the end.
About.com has a Mitre Tutorial you can check out
Here are some nice mitered square knitting patterns links for you to check out:
Mitered-Square Rug (PDF) from Interweave Knits
Psychedelic Squares Afghan -- so fun looking but knit using stockingknit so the technique is a little different
Spike - Mitered Lace Scarf from Winter '05 Knitty