Hello everybody! This is a post by guest writer DragonBringerX, with 7 rules on how to run a sandbox game. I thought it was great, and I'm sure you guys will too. You can find the original post here (used and edited with his permission).
1 - Do not tailor the game to the players or their characters. Players should be encouraged to play whatever they want. If they come to an obstacle they cannot overcome, let them fail or come back to it later. This will influence players to be more well rounded in their character creation, or at least as a group.
2 - DO NOT ASSUME ANYTHING, EVER! I cannot stress this enough. DO NOT require a skill check [OR ANY TYPE OF ROLL] to move the story/plot/mission/quest further. If the players need to find a clue to move on, they simply find it. If not, then call for a skill check. Lets use the locked door as an example. The 3 obvious choices are lockpick (if able), bash (if able), find keys (if able). If there is a possibility they could fail all 3, it should not be required to get through that door. Instead, the door should have something else behind it that helps the players if they do succeed. Such as a shortcut, treasure, or possible just an empty room (whatever the situation calls for).
3 - The 3 clue rule. When planing a mission/quest/adventure, always try to leave at least THREE (3) possible conclusions to said mission/quest/adventure or 3 clues to solve it. If the players feel/look like they are getting stuck, or are bored, or lost, give them clues. For example, I had a player get stuck trying to move through the sewer under a ruined city. Climbing up and moving through the city was always an option (a dangerous option, but an option). Never assume the players have a particular skill or a particular skill high enough (or roll high enough) to accomplish anything (see above).
4 - Be FAIR. This doesn't mean what you think it means. It means, in a TRUE sandbox game, if a character gets killed by a goblin, the character gets killed. If they fail a save, they fail. If they succeed, THEY SUCCEED. Plain and simple, whatever the outcome maybe, move on.
5 - Do NOT become attached to anything! When fleshing out your world, do not become attached to it. I know this can be difficult, especially for your NPCs you worked so hard to be cool. But here's the truth. They ARE NOT COOL. No NPC can, will be, or should be cool EVER. I use a rule of thumb that if you think the players will like an NPC, the players will try and kill them. And you need to be fine with that, and allow it to happen (because it will). This includes NPCs, locations, items, and even plot-lines. That's right, quests. Your players will hate your quest you spent 5 hours on, and love the one you spent 5 minutes on. This leads me to my next point...
6 - 10 Minute Rule. When planing a mission/quest/adventure/anything (except the world), do not spend more than 5 to 10 minutes on it. Jot down a few bullet points, important names, 3 clues, reward...and DONE. That it. Trust me, your players wont even realize.
7 - The WORLD. This is the ONLY exception to the above rule. Spend days, weeks, or even months planing the world. Create towns, cities, economy, factions, companies, governments, gangs/criminals, guilds, schools, religions, magic, and the meta-physics of your world/universe. The one thing that makes any sandbox game work is a world for your players to explore, become immersed in, and do stuff in. So, after you have a world (or a small piece of it) fleshed out, plan plenty of things for your players to do in it; and expect for about 1/3 of it to NEVER get used. See rule 6 above. I recommend creating about 10 to 20 missions for your players to pick and choose what they want to do. If the players come up with their own thing...roll/role with it.
In the end, NEVER say no. Says yes, asked for a roll/role, and decide the outcome. Sometimes players may want to do things or ask for things they cannot do or have yet...that's fine; just tell them "not yet".
A few more pieces of advice. Keep DC's low, use NPC classes ALOT, do not overuse Core classes, keep treasure rewards low but frequent, give players plenty of options, and above all BE CLEAR WITH YOUR PLAYERS, BOTH AHEAD OF TIME AND DURING THE GAME!