The kind of oversideboarding @diophan is talking about is really common in vintage (probably in magic). If you can get a handle on that you'll be ahead of the curve. Just make sure you follow it to it's natural conclusion. If you don't have enough cards in your deck to take out in order to fit, say, the Echoing Truth ... then that Echoing Truth could have been something more useful in another matchup. Adding cards to a sideboard without considering what you need to cut for them is a critical error I see people make all the time.