Buy your tickets online at the exact moment that tickets go on sale. Waiting in line outside of a box office can be very boring and unless you are one of the first people in line you may not get the tickets you want.
Check Ebay, craigslist or similar sites for floor tickets if you could not get the tickets before they sold out. Be prepared to pay more than standard price though, as some people buy tickets simply to sell them for more on Ebay.
Go to the venue on the night of the performance and see if any "scalpers" have tickets. These people often sell tickets for 2 to 4 times the original price, however. Check your local laws to see if scalping is legal in your area.
Check pre-sales. Often, ticket agencies will open online for early sales before tickets are available to the general public. If you check the website when the show is announced, the pre-sale times and dates are usually available. The pre-sales are hosted by a variety of organizations including radio stations and credit card companies. This is a good way to get tickets before sales are available to the general public which will increase your chances of getting floor seats.