Log onto the website where concert tickets are available from (typically TicketMaster and LiveNation). Select the artist and the venue you want to buy the tickets from, then select the number of tickets you want. An option is available for selecting a specific section at the venue, or to choose "Best Available." The latter option is going to locate the best tickets still available to the concert, rather than search for a specific area at the venue. Once you submit the information, the website scans for the tickets. If you are not happy with the tickets, select "Release Tickets" and retry. Often times, when tickets to a show first go on sale, individuals that land the best seats either have their credit card declined or don't submit the purchase information in time (you are given just a few minutes to complete the order form). When this occurs the better tickets are released back to the public. If you continue to refresh and retry, you may land these better seats.
Contact your credit card company. Several higher-end credit cards, such as the American Express Platinum/Centurion and the Visa Black, have 24-hour concierge service. These services have inside access to better seats (occasionally some of the best seats are reserved for individuals holding these cards). If you purchased the tickets directly through the credit card's service they may be able to upgrade your current ticket to something else still available.
Sign up for the musician's fan page. Many musicians sell tickets to individuals in the fan club first. This increases your chance of getting the best seats in the house. If you have already purchased your ticket through a different source you probably can't exchange it, but you can sell it for face value at the venue or through an online ticket brokerage firm, such as StubHub.com.