Currently (through July 12, 2013), in Constructed formats, you’re allowed to play with a sideboard that is either 15 or 0 cards exactly. Moreover, if you swap cards between your main deck and sideboard, you have to exchange them on a one-for-one basis.

Both of these rules are being changed with the Magic 2014 rules update. Your sideboard can be any number of cards from 0 to 15, and the restriction on swapping cards one-for-one has gone away. As long as the total number of cards in your sideboard after sideboarding is 15 or fewer, and your main deck is at least 60 cards, your deck and sideboard are legal.

The main benefit of this reduces the “feel bad” moment when you accidentally sideboard in more cards than you take out, which previously would result in a Game Loss. Now, doing this (either accidentally or intentionally) is perfectly legal.

Here are some older tips that have been updated for the new Sideboarding rule:
http://blogs.magicjudges.org/rulestips/2011/04/presenting-your-sideboard/
http://blogs.magicjudges.org/rulestips/2011/02/constructed-vs-limited-sideboard-use/
http://blogs.magicjudges.org/rulestips/2012/06/in-limited-events-without-decklists-you-can-change-your-deck-between-matches/

Today’s Tournament Tip written by Paul Baranay