I have to admit that descent planning does my head in at times aswell. I know it is simple in theory etc but when you get different people sitting next to you advising you to try it in different ways I get confused.
I love it when the Captain allows you to experiment and learn for yourself which is the best way.
I like to try and keep it simple by inputting as accurate winds as possible into the descent forcast page and descending in Vnav, starting descent in the ball park of 25nm before calculated TOD.
Vnav the other night was not proving too accurate so I used Level Change and had the same issue, the plane was just gaining speed even with speed brakes at 50 percent. It does not happen often, but each plane flys differently.
I try to avoid using V/S as much as possible, only using it for small changes, but some pilots swear by it and use it 95 percent of the time, even though you have no speed protection........ that always gets me. Each to their own.