Having lived in the Philly area for a decade I watch a lot of the Eagles as a perennial playoff contender:
1. Hired a coach/GM off a proven coaching tree (Reid from Holmgren, subsequently divested of GM title)
2. Had an Overall Vision and STUCK WITH IT and drafted around it (WC Offense; Pressure Defense)
3. Drafted early on a strong armed, mobile, coachable franchise QB in McNabb even though they got laughed at and booed
4. Consistently address the O and interior D lines early and often in the draft, too many examples to list (NEVER running backs (Westbrook 4th round), linebackers (name one) or safeties (Dawkins an exception in the 2nd before Reid got there)).
5. Trade down when they see no value, not afraid to stockpile draft picks (see budding star Jackson, their first pick after trading down, in the second round)
Like Marv said, simple but not easy.