Become a Top Elite member or fly first class. Delta customers who fly a certain amount of miles per year are grouped into elite categories, with Diamond Medallion the highest level. Those passengers earn Top Elite status and are allowed to board the plane first. If you fly first class, you are also allowed on the plane as part of the first group of fliers to get on board. If you are in the first row of coach just before the division into first class, that also qualifies you for preboarding.
Fly Delta frequently. The second group of fliers includes Delta and Northwest frequent fliers. The merger of Delta and Northwest essentially means that all Northwest frequent flier miles were converted to Delta, so if you have enough frequent-flier miles from both airlines, that would qualify you for Zone 2 preboarding status. It pays to take part in Delta's partner programs, which include relationships with hotels, rental car companies and restaurants.
Participate in Delta's Sky Priority program. The priority program allows you to call an exclusive reservation line and check in for your flight using check-in areas dedicated for frequent-flier passengers such as yourself. Bonus services offered with Sky Priority include express baggage service and priority security screening. That will enable you to beat the long lines for regular passengers going through checkpoints. Sky Priority passengers are considered the top level of flier and board flights first.
Belong to a flying partner of Delta. Most domestic airlines do not take part in this service, because it is considered competition, but international airlines such as Air France (airfrance.com), Alitalia (alitalia.com) and others allow you the chance to board with priority. If you are arriving from an international destination and need to use Delta to connect with your destination city, membership in that international airline's frequent-flier club will earn you bonus points on board Delta flights.