In their excellent book The Big Book of Bible Difficulties, Norman Geisler and Thomas Howe explain:
"Jesus actually cursed the fig tree on His way to the temple as Mark said, but this does not mean that Matthew's account is mistaken. Christ made two trips to the temple, and He cursed the fig tree on His second trip.
Mark 11:11 says that Christ entered the temple the day of His triumphal entry. When Christ enters the temple, Mark does not mention Christ making any proclamations against any wrongdoing. Verse 12 says "Now the next day," referring to the trip to the fig tree on the way to the temple the second day. On this day, Christ thew out those buying and selling in the temple. Matthew, however, addresses the two trips of Christ to the temple as though they were one event. This gives the impression that the first day Christ entered the temple He drove out the buyers and sellers as well. Mark's account, however, gives more detail to the events, revealing that there were actually two trips to the temple. In view of this, we have no reason to believe that there is a discrepancy in the accounts." 
This explanation demonstrates why it is important for both believers and unbelievers to use sound principles when evaluating alleged Bible contradictions.
Courage and Godspeed,