Paul,
The best way to approach VOR questions is to always draw a picture. And the basic concept you need to clear up in your head is that the radial you are ON is completely independent of the heading you are flying.
To take the example you gave: "If I am on a 90degree radial FROM a VOR, what bearing do I fly (assuming no wind) to arrive at the VOR?What happens after I fly over the VOR? Do I have a 90 TO on the VOR now? Do I have 180 FROM?"
To deal with this, draw a picture, which is basically a compass rose. The VOR itself is the dot in the middle, the 90 radial is a line out from the VOR at 90 deg, the 270 radial is a line out from the VOR at 270 deg etc etc. Now place your aeroplane ON the 90 deg radial. It should be immediately obvious that the heading you have to fly to get TO the VOR is 270 deg. Imagine now you've overflown the VOR. Place your aeroplane on the picture again - it's on the 270 radial now. What heading would you now have to fly to get TO the VOR? 90 deg. What heading would you have to fly to get away FROM the VOR? 270 deg.
TO and FROM are as simple as that - what HEADING must I fly to get TO the VOR? and what HEADING must I fly to get away FROM the VOR?
Hope that helps! It's certainly torture coming to grips with it, but once the penny drops you'll wonder what you ever found confusing
