Hi Gill,
I took a class on diagonal knitting at the Northwest Regional Spinners Association anual conference. The two things that were really helpful are:
1. Any gauge that knits up square such as garter stitch; seed stitch, moss stitch, slip stitch, mosaic stitch, will work up fairly square on the diagonal. Increase at the beginning and end of the right side row and work the wrong side row plain. Work a swatch to see this.
Eclectickat
2. Stockinet stitch is not square! To compensate for this increase on each end of the knit row and in addition increase at the beginning only of the purl row. This gives three increases instead of two for every two rows.
Before starting work up a swatch of your pattern stitch and see if it works square or is off a bit. Once you know your pattern you will know if any extra increases are needed to square it up.