I DM a campaign that's now around level 30 (started at level 10 4 years ago; we wanted a change from low-level play and to see what the game was like at higher levels.), and both my players and I are enjoying the campaign more for each level.
So far I've had no problems coming up with plots and challenges; quite the reverse: I find that the more powerful the NPCs are; the more resources they have, the more complicated plots I can devise for them.
As it has already been said repeatedly, one secret to challenging your PCs is to know the rules for monster advancement; HD, templates, class levels... I never use stuff straight out of a book at this level. Another important thing is to know what your monsters and NPCs are capable of, so stat them out
fully! At this level they have a lot of special attacks, special qualities, spells etc., and the DM needs to take full advantage of these to make encounters interesting and challenging.
Admittedly it's an awful lot of work for me, but that's largely to do with my style of DMing. I like to have at least half a dozen loose threads and plot-hooks dangling in front of the PCs and then let them choose which hook to bite (I hate railroading!
![Devious :] :]](http://www.enworld.org/forum/images/smilies/devious.png)
). This means that for any given session I have NPCs, monsters, strategies and locations for roughly six sessions ready!

It's ok, what I don't get to use one session, I usually get to use later on.

Thankfully, I have a job with a lot of down time, so I can bring my books and notepaper with me!
