I'm by no means an expert, I'm sure there are others here who have a better answer to your question than me, but I'll tell you what I do: If I'm shifting down (easier), I first shift the back up one or two gears (harder) so when I next shift the front ring down I don't spin out of control. If I'm shifting up I shift the front first, then the back to whatever the slope warrants. I hope this makes sense!