For keeping track of shaping, such as armholes, get yourself a whole bunch of removable stitch markers. They don't have to be "formal" stitch markers, paper clips, safety pins, or bits of scrap yarn in a contrasting color will work. Every time you make a decrease/increase put a marker in the last stitch of the row. That way it's really easy to see if you need to work shaping again (such as every 4th row) or if you have done enough of them yet. If you have enough markers to leave them in when you are knitting other pieces it can also help you match up the shaping later when you seam things.
Oooh! I'm definitely going to try that. I'll save my scrap yarn to mark stitches. Thanks!