I was there 3 years ago. If your idea of a holiday does not
include things like tour to cultural/historical places (basically
propaganda sites to the rest of the world) with nothing but
statues, portraits and grand eulogies by the guides about the
greatness of the two Kims, you're better off looking
elsewhere.
Just in case you have no idea what that place is about, it's not
exactly a free-and-easy holiday where you can do what you want.
Anyway, there're only 2 ways to get in and out of DPRK - train from
Beijing or flight from Shenyang.
For a start, joining a tour is a must. As far as I
know, Koryo Tours (someone has given a link above) is the only
agency conducting tours to DPRK - all packages offered by foreign
tour agencies are basically conducted through Koryo Tours. Not only
are you not allowed to wander on your own as you deem fit, you
basically cannot remain out of your guides' sight (yes, don't even
think about leaving your hotel for a stroll after dark although
it's not like there's anything to see when there're hardly any
lights after midnight).
That said, you might want to check out the Koryo Tours website for
the various packages. If you're thinking of going with like-minded
friends, you might want to approach Koryo Tours with an itinerary
(you'd need to do your own research first as to what places you
want to visit) of your own for independent tours (which was what I
did). Although this is a little more expensive, it certainly
catered better to my needs than joining a large tour group
IMO.
In spite of the restrictions and the bureaucratic nightmares, I'd
certainly have to agree with someone posting on this thread
suggesting that'd be a holiday of a lifetime - you don't get an
experience like that anywhere else on the planet.
If you need any information, feel free to PM me. 
