Becoming better at sideboarding
In my experience, the hardest part of sideboarding is to know what cards to take out. There are some obvious cases like removing creature removal against a deck without creatures and such, but they're not the most common case. Sometimes you change one card for another with the same function (swapping Terror for Diabolic edict against a black deck), that's easy too. But what do you take out when you need grave hate against a deck with no obvious bad cards in your own?

Focus on what cards will make you win the game?

It really helps to know you deck well when you're going to sideboard. What cards are the core of your game plan. How many creatures/disruption cards/mana sources/sacrifice outlets/whatevers do you optimally want? And how many can your deck still manage to play well with. How is your mana curve? Can you manage that 4-cost spell that's better or do you need the cheap one. Make sure not to focus too much on hampering you opponents game plan so that you ruin your own!

No matter what cards you decide on, don't forget to playtest! That's what really shows you how the deck plays after you've sideboarded. 

Tip: A common test strategy when you don't know which of 2/3 cards you want to bring in is to play them as split-cards when you playtest. You get to chose which one you play during the game and after a few games you have a good idea of what you want.