I have been living in Philadelphia for nine years now, and while I can tell a Philly accent when I hear one, I cannot, for the life of me, figure out how to do it myself. Something about the vowel system gets me all tripped up, and I end up sounding like cockney Tony Soprano.
Part of the problem is that there aren't any well-known popular culture characters to imitate. You know, if you want to do Minnesota, you channel Fargo. To do Boston, you put on a little Good Will Hunting. But who do you imitate to "do" Philadelphia? The accent rarely shows up in movies or TV, even when they are explicitly set in Philadelphia, as this New York Times article points out.
I've gotten a little better at the accent, or at least at understanding why Philadelphians sound the way they do, from watching Sean Monahan's PhillyTawk YouTube videos. They're fun and entertaining, but they also break down the linguistic concepts behind the accent in an accessible way. Here's the explanation of the split short-A system. It helped me realize that to pass for a Philly native I will need to work on mastering the sentence "halve the hoagie, then have half."
And while I knew about that old shibboleth where the Eagles become the "Iggles," I never thought about how a Philadelphian would say "Craig's essay about the effect of the Black Plague on the medieval Hague was pretty vague."
If you've been mystified by the Philly (or South Jersey, or Baltimore) accent, all the PhillyTawk videos will help. We've all got to do what we can to keep up appreciation for this distinctive American dialect, at least until natives Bradley Cooper (at 1:15) or Tina Fey (at 3:40) bring it to the big screen.