As a rule, I've found that in 90% of gamebooks the "GM Advice" section is useless filler, generic pablum that no one but the rawest of beginners would find worth reading, right up there with the "what is roleplaying?" essay that often games that only the most hardcore of experienced gamers would ever bother to buy still think they're obliged to write up.

But once in a while, you get a gamebook that has a "GM Advice" chapter that blows your mind.

In my case, it is Amber.  The best, absolute best GM advice section(s) ever.  That game MADE me a GM, and you can pretty well define good vs. bad Gamemasters by the spectrum of how many of the ideas in the Amber GM-advice material they have already clued in on, whether from reading Amber or independently.

Another brilliant one was the Call of Cthulhu D20 book.  The absolute best "how to run horror" advice I've ever seen in an RPG.

What other ones hit the mark?

RPGPundit

Rifts: Adventure Guide.

It's full of awesome!

Go check yourself, it's a hundred pages of pure GM advice from different GM-philosophies. Bill Coffin really shared some great stuff, as did Erick. And guess what? Evin Kevins "a dungeon really is a flowchart" article has lots of cool stuff in it.

Listen Up You Primitive Screwheads - Cyberpunk

and this

Yeah, I thought of Jeff's blog, too.

Idunno.  I thought the Star Wars 2nd Ed. Revised and Expanded book had some good GM advice; Theatrix did, too, but I let it lead me down the path of Gaming As Art and I think I got stuck there for too long, so go with caution.

CoC d20 is loaded with good stuff. M&M2e has a good section too- although they should have spent some time on Kirby/cosmic style stuff.

The D&D Rules Cyclopedia has some pretty good DM's advice:

(on page 262):

The Most Important Rule: Be Fair.

Yes. The real Golden Rule:

Be fair. Follow the rules, make the players follow them. Ensure everyone has equal opportunities, without equalizing them.

Quote from: Dr Rotwang!Theatrix did, too, but I let it lead me down the path of Gaming As Art and I think I got stuck there for too long, so go with caution.

That could've been circumvented if you used the "Ironwood" source book, which wasn't all that "artsy".

I'll have to give a nod to Amber, too. The rules for combat were quite nice and help a GM describing things even when he doesn't actually decide them...

"Robin's Laws of Good Game Mastering" got some mixed results for me. I'm really not to fond of all the "player archetypes" stuff, as I've yet to find someone who's clearly in one category. But the flowcharting of adventures was pretty useful.

The Dying Earth RPG has some nice guidelines for creating adventures, listing all the basic elements a Vancian episode should have ("Outlandish Cuisine") and ways to achieve them. I find myself using some of that even in more serious games...

Doc, Lawbaga, thanks for the kind words!

S. John Ross is a good source for awesome GM advice, particularly his Narrator's Toolkit for LUGTrek and the Risus Companion.  Aaron Alston's Strikeforce rocked my world as far as GMing advice goes.  Most of that stuff made it into the most recent edition of Champions.

Best advice ever?

"Bring your own shit."-T Willard

That means dice, snacks, books, character sheets, soda, smokes, whatever.

Don't be a goddamn leech.