In no particular order.....
- Do not get bogged down in details. GURPS provides a wealth of rules and material to let you quickly adjudicate almost any action and you can bet the players will go off the rails and in unforeseen directions from time to time.
- NPC: Do not fully stat minor NPCs or worry about point totals. A common mistake is to build an NPC as if it were a fully developed character. This is not needed and if the NPC does not get as much screen time as expected it may have wasted a lot of time. Do not worry about point totals, just give them the important skills and attributes as needed for planned adventures and flavor with personality.
- NPC: Keep a stock of ready to go NPC cards or files with basic info such as personality and key skills. Things like Generic Thug, Bartender, Generic cop, etc. can save you time during the game. Also do not be afraid to change names and a few minor bits and reuse them. In fact sometimes this can be an "inside joke" among the group and add to the campaigns fun. Do not overuse them though, show some imagination for major NPCs.
- Improvisation: When in doubt roll and shout. Things will come up where your not sure on the specific rules or modifiers for a situation. Looking things up during the game can mess with the flow of play, so if it will take too long just make a judgement call. Let the players know this and look it up later, sometimes a player may know the ruling and save you time. Other times your guess usually good or close enough and if you have the trust of your players in being fair it should work out fine. Even if you have to inform them that you made a mistake and it should have been harder or easier they will understand and appreciate the heads up for next time.
- Keep Notes: Things will come up during a game that you should note for later, these may be plot ideas that you or the players think of during a game, rulings made, NPCs improvised, etc. The notes will help you not forget something useful and plan better for later.
- Plan out the details and rulings in advance. Write down page references for things that you expect to come up during the next game or on an NPC sheet for things they use you may not be familiar with. This can save searching during the game.
- Give every player some screen time during a game. Keep everyone involved as much as you can and if one player is taking up a lot of time make sure it is useful, important and the rest of the group enjoys it. If a specialist (Face man, Hacker or Planner) is taking up a lot of time the others are likely to get bored. If the player doing it is entertaining and the action exciting or interesting objections will be less and take longer but be mindful of the other players. Try to keep everyone involved as much as you can.
- Keep in mind player abilities and disadvantages. These are all potential plot hooks and if it is on the players sheet they likely think its important or interesting. So make sure they come up, even the minor stuff.
- Reward good roleplaying. Good roleplaying can add fun to the game for everyone so reward it when possible. However be mindful of some players monopolizing game time.