These days life is made much easier through the propagation of online ticket agents with assured allocations for Bara matches whereas buying Barcelona tickets in advance was quite a difficult process until a few years ago. However, with so many vendors making Barcelona tickets available online there is a new problem; which is the best and how to choose between them?
Firstly look for companies who have been involved with ticket sales for a while and it should be easy to get in touch with them, for example Simply Barcelona Tickets. Since they have achieved vast experience selling online tickets there is far less possibility to experience problems with your order and any issues you have will be dealt with quickly.
Then you must make a decision as to what are the most important criteria for you. Do you require the best tickets in the house regardless of the price? Can you afford to be present at "el clasico" between Real Madrid and FC Barcelona? Or do you just want to see the match while you're in Barcelona and just want the cheapest seats in the house. You must be conscious that ticket agencies will set a markup on the face value, however the benefit you get from them may make up for this in terms of saving time while in Barcelona.
If your friends have any knowledge of online ticket agents for FC Barcelona matches then it can also help to ask them. You may be better off using one of the FC Barcelona specialists whereas there are several companies that sell tickets for all kinds of events Specialists have better access to tickets and by focusing on one sport they really know their business inside out. Yet, if you have formerly bought tickets from a general ticket agent and have been happy, then they are worth considering again.