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I am thinking of trying to run a planar campaign and would like to know if anyone has any tips for running one. I already have a copy of the Planar Handbook and can get just about any other book that I may need.
Familiarize yourself with the general theme if each if the planes. Be prepared for you're players to go places you weren't expecting. Especially if they have access to their own means of plane shifting. If they will be primarily portal bound, you have a bit more freedom to control where they go.
Otherwise, come up with plots based on general themes of the planes. Make up your own special unique locations, or expound upon some of the example locations like The Catalogues if Enlightenment or the City of Brass. Also, it might be worth while to consider ways to temporarily survive in areas that are hostile to fleshies, like the Elemental Planes or some of the harsher outer planes like Cania or the Plane of Shadows. Temp warding objects that grant 24 hours of protection introduce timed quests and add drama while allowing your PCs to explore some of the more abstract (and fun) planes.
A beholder’s favorite foods include small live mammals, exotic mushrooms and other fungi, gnomes, beef, pork, colorful leafy vegetables, leaves, flower petals, insects, and birds.
Well, apart from the Planar Handbook there is also the 'Manual of the Planes' -which is D&D 3.0 but still has some useful information.
Then there is obviously the AD&D Planescape stuff, not useful for crunch but has the most delicious fluff and artwork. The first title that comes to mind there is 'The Planewalker's Handbook'. For a complete list, see here.
Of course, there is always the fun of making your own planes. Or even using ones other people made. I've heard the Semi-elemental plane of ranch dressing is quite fun.
On this note, Libertad's reimagining of the standard planes is quite good, including a nice overview of each plane's characteristics and landmarks, some possible adventure hooks, and correcting various inconsistencies in standard fluff.
Two other source books that I think would be worth a glance before starting a planar campaign would be Fiendish Codex I and II. The only deal with two planes, but they're so detailed and varied that they'll give you a lot of good ideas.
Make sure they have the ability to escape from where they're being sent. This may sound dumb, but if they're say in the plane of fire and don't have a way to get home if they get badly injured, the campaign may be over.