The SJ Games website has a page of Freelance Writing Tips. Point #1 on that list of tips talks about the need to balance storytelling skills with game-rule design skills. I feel that there's more that could / needs be said about this topic and will endeavor to do so here.
Role-playing has been described in one way as a collectively-written story; many authors contribute in telling a tale. The players "write" the parts of their characters, and the GM "writes" everything else and serves as Editor in Cheif. As you can see, there's a bit of an imbalance as the GM needs to "write" quite a lot. The following are hopefully some guidelines that can help the GM develop his storytelling abilities.
Your ability to run adventures will be greatly enhanced by an understanding of Joseph Campbell's Hero Monomyth. Elements of said Monomyth are given here, with some additional items that tailor it to an RPG session.
Another excellent essay on this topic that goes into much more detail is Mythic Structure in Role-Playing Games by J. Patrick McDonald
Truthfully, a lotta players are just spoiling for a fight. If you don't give them one soon, they'll probably make one of their own, likely in a real inopportune place that will interfere with the progress of the adventure. So, it's often a good idea to have a planned (and innocuous) fight up near the beginning of the adventure, like say, a bar fight, so the players can get it out of their system.
While this may seem like a trivial item, it can really make things a lot smoother if you have a ready list of names for NPCs. You don't even have to match up all the names exactly, just have some on hand.
If you're playing a Victorian-era game like us, take a look at this page which contains a copious listing of names of characters in Victorian-era reads, from dimestore novels to penny dreafuls.
For an example of a table-driven approach to name generation, have a look at Jared's Fairy Jamboree adventure.
If you have ever listened to Books on Tape (aka "Spoken Word") you'll know that the best readers are the ones who make an effort to make sure that each character in the book has a distinct voice. A good GM should do the same thing.
In short, the two things that each NPC needs is:
Important NPCs should have more than one quirk.
It can spice up your adventure immeasurably if you can have some pictures of NPCs and buildings that your players can look at. If you don't fancy yourself as an artist, look in some books or prowl around the Internet, there's got to be a picture or three you can use somewhere. Being able to see what the police cop looks like or see what the old castle looks like can really make the story more engaging to the players.
Tricks & traps are like jalapeno peppers: if you toss in a few here and there, it can spice things up; if you throw in too many, it just burns. Make adventures a little more intelligent and varied, mm 'K?
Dice are an important part of roleplaying games because they introduce an element of randomness and test your probability of success or failure. However, too much rolling can make a game quite tedious. With that in mind, you should look for ways to reduce the number of die rolls that have to take place. Some examples include:
The occasions when you really want to cut down on rolling are:
In addition to being the lead writer / Editor in Chief, the GM needs to be able to recall and arbitrate on game system rules when the occasions arrise. The alert reader will note that some of the advice given here contradicts advice given above in the "Storyteller" section. Sometimes you need to be the Storyteller, sometimes you need to be the Rulemaster. A good GM knows how to balance those roles and when it's more appropriate to play one or the other.
There's no magic bullet to being a good rulemaster, you just need to learn the rules and keep a lot of information in your head. You should know at least the following: success rolls, skill use, athletic skills, hand-to-hand combat, ranged combat, vechicle rules, damage, injury, fatigue, recovery, stun, quick-draw / aiming and other gun-related rules.
It can seem a bit daunting to keep track of all that stuff. Most RPGs will provide you with some ready-reference sheets and tables that you can keep on hand so as to quickly look things up. Avail yourself of these. Make copies. Keep them handy.
Write up interesting tables. Some obvious ones are things like encounter tables, loot tables, traps tables, and the like.
More interesting are "rumor" or "information" tables. At the the introduction of an adventure, the PCs are given a mission by some important NPC (go rescue this person, go loot this castle, bring me the head of the three-headed goat, etc.) In the middle of the adventure, the PCs are crawling through the catacombs / caves / dungeon / whatever actively in pursuit of whatever was spelled out in the intro. Somewhere between the beginning and the middle is a part where the PCs need to do some legwork to find out what's going on, what exactly they need to do, how they should prepare themselves, etc. This is the point in the adventure that calls for "rumor" tables.
Fortunately, in the GURPS system there is a built-in mechanism for rumor tables: The reaction roll system. Here's what you do: draw up your own reaction table divided into the Poor, Neutral, Good, etc. sections and put rumors in each section. The quality of the rumors should be reflected in the section where they are found, with irrelevant / misleading rumors in the "Poor" category, average rumors in the "Neutral" category, and valuable / relevant rumors in the "Good" category. As the PCs are wandering around town talking to people, roll on this table when they meet someone and give 'em the rumor.
For a good example of a rumor table, have a look at Jared's Fairy Jamboree adventure.
Occasionally there will be situations where the PCs engage in some sort of non-combat related contest. Standard rules typically say "Each makes a roll, best roll wins" but this does not account for contests that could be long, drawn-out affairs i.e. gambling, drinking, dart-throwing, riddle-telling, etc. Some advice for handling contests is given on this page.