Uncanny, I just ran into this one when installing shoe molding. For some reason my brain could not envision the final mitred corner and how to translate that to saw cuts. Here's what helped me:
1) Look at how some existing trim is installed. Hold your new trim up to envision the angle. 2) Get a mitre box, if you don't have one already. (Couldn't tell from your message.) It has slots where your saw goes in to keep the angle straight. 3) Make sure you cut the trim face up, as it will appear installed. Yeah, it sometimes gives a raggedy cut, but it's MUCH easier to envision the angle this way. I've sanded the edges of a ragged cut with good results. 4) Most corners (at least in my house) are not exactly 90 degrees. Keep this in mind when cutting trim. I usually cut a single piece at a time, dry fit, cut the second piece, dry fit, etc. This way, you can adjust the edges to fit the corner. If it doesn't fit, you haven't nailed anything down.