13th Age is a different beast than other d20 games, and GMing it effectively can take some getting used to. What follows is a list of tips to help ease the transition. I don't claim to be a spectacular 13th Age GM by any stretch; indeed, a lot of the items on this list are things that I'd like to strive for myself.
Tip 1: Get comfortable with improvisation. But first thing's first - accept the fact that you'll have to improvise if you take advantage of the system's strengths.
Tip 2: Don't be afraid to lean on your players. 13th Age gives players a lot of narrative power, allowing them to shape the campaign into their own ideal playground. You're still in charge of the details, but if you're drawing a blank your players will probably have some suggestions.
Tip 3: Fail forward. Sure, a lot of games embrace this philosophy, but it just so happens that it's important to 13th Age as well. As an example from the last session I ran: the Cleric wanted to identify these glowing lichens and glowing mushrooms in a dungeon passage. After failing the Intelligence check, the player said "well, I guess I have no clue what they are." To which I responded "You don't know their specific properties, but they look a lot like something you've heard in the stories of drow. Drow avoid them." Failure, and my on-the-spot thinking, actually generated a more interesting answer than if he'd succeeded and I simply told him what the fungi were.
Tip 4: The Mephistophelean Bargain. This is one of Rob Heinsoo's trademarks. It's a variant of the "yes, and..." school of thought. When a player suggests something, you shouldn't just say "no" because that's not very interesting. Even if it's something outrageous, you should give them an option to go with that - but with a commensurate price! This can either be a gambler's bargain, where the rewards for success are great, but failure offers a very severe risk, or it can simply mean accepting consequences along with what is asked for.
Tip 5: Minis are very helpful. Yeah, this is a gridless game and you can easily play it theater of the mind style, but minis make it extremely easily to quickly resolve any logistical questions the players might have, and everyone's on the same page in terms of where everyone is.
Tip 6: Ask leading questions, particularly during character creation. My players have a tendency to start out with pretty generic backgrounds. I wish I'd pushed them more to be a bit more creative, but it's starting to get refined out in-play all the same. The more specific your players get with their narrative mechanics (Backgrounds, One Unique Thing, Icon Relationships), the richer the game experience will be.
Tip 7: Now that you've encouraged the players to give you a bunch of built-in hooks, use them! The simplest and most overt example of this is using Icon rolls, but make sure you're weaving in elements from the character's backgrounds and OUTs as well. If you're ever stuck for ideas, turn to the character details. Players like when the plot becomes personal, so you can turn a dead end into something interesting.
Tip 8: Be prepared to house rule. The game's math is extremely transparent, so take advantage of that fact! I've had players complain about their classes, and since this is a story-focused game the players need to be invested in their characters. If there's a change that can be made to improve the experience for them, try it out! For example, my group found the Barbarian to be too weak so I'm more than happy houseruling it. Likewise my Cleric is getting bored with having only 1 ranged at-will spell, so I've set him to work homebrewing a new one (pending my approval, of course). You can directly modify the class mechanics, adjust the experience with significant magic items, or even use story rewards (like 4E's grandmaster training or divine boons).
Tip 9: Keep monsters interesting by sending them in in waves, having them play differently when staggered, or giving them abilities tied to the Escalation Die.
Tip 10: Create interesting items for players to spend their gold on. Ok, so not all campaigns have a big gear focus, but even though I'm one who likes to gloss over gear I still find the consumables list lacking. Potions (2 kinds), oils, and runes will get old fast. I've made a larger list of potions, and I'm also going to start keeping my eye open for story-based ways to make the PCs' lives easier if they're willing to shell out some cash.
Tip 11: Keep the terrain fresh, and give players a lot of non-standard options during combat. 13th Age doesn't have a robust tactical engine in the sense that, say, D&D 4e did. The advantages of this is that it's faster, and more flexible. Use Talents like Swashbuckle, Terrain Stunt, and Vance's Polysyllabic Verbalizations for inspiration (but make sure these options come with a cost, so as not to make those talents obsolete!). Players won't take the bait despite your hints? Have the monsters pull some crazy stunts for a less subtle approach.
Tip 12: Steal! Steal ideas from books, movies, tv, whatever. This is something that Chris Perkins emphasizes all the time, and it's all the more appropriate in a more narrative-centric game like 13th Age (compared with D&D). The more engaging and rich your story is, the more satisfying all of those story-based player resources will be. Of course disguise anything your players might be familiar with in a fresh set of clothes. In other words, don't copy+paste, but use familiar media as a starting point that you can add to. It can add a lot to the experience if you have a character in mind whenever you're playing an NPC (or an Icon), if nothing else than for consistency in how you portray them.
Tip 13: Have fun! Ok, so I started running out of worthwhile ideas, but this list just BEGS to have 13 tips, and having fun is really important (albeit obvious).