A common problem I observe is that the players start flooding the GM with questions regarding rules during an encounter (within and outside their respective turns). From the perspective of a GM, how can I reduce the damage/clutter/blockage this can cause but still give the players the knowledge they need to decide what actions they will take during their upcoming (or current) turn? What about outside of combat?
Give them an immediate judgement call, then look up the actual rule after the session and use that from then on.
When the player asks a rules question, quickly decide for yourself what you think the answer ought to be. Make it clear to the player that this is not a final ruling, it's just what you're going to use for that session in the interest of keeping things going. If they can find the relevant rule in the book to show that you're wrong in 30 seconds or less then go with the official rule. If they can't find the rule, or it's a matter of interpretation (read: arguing), then use your on-the-spot ruling for that session, then after the session look up the rules, discuss it with the player, and come to a final decision that will apply if the situation ever comes up again.
Note that this can be problematic in cases where the rules are unclear (or nobody can recall them) and the impact of a judgement call is extreme (whether a PC lives or dies, for example). In those cases it's probably better to look up the actual rules & come to an agreement during play, but for any other discussion the temporary ruling should speed things along. Alternately, as SevenSidedDie pointed out, you can just rule in a manner that preserves the status quo or that has a less permanent result.
Again, you should explain what you're doing and why to your players; while most players will be perfectly happy with sacrificing rules arguments to speed up the game (except for the hardcore rules lawyers), if you don't explain your policy to them it can just seem like you've decided to resolve everything with arbitrary DM fiat.
Edit: This system should handle questions from normal players (who don't tend to ask tons of questions, and should be happy with a system designed to speed up play) and from rules lawyers (who will realize they're not getting definitive rulings and quit asking questions during play; be prepared for them to pester you for clarifications/rulings between sessions) as long as you make it clear that you are absolutely not willing to argue about the rules during play. There is 1 type of player who will likely continue to ask a great many questions under this system: new players that haven't read the rules (whose questions will generally be "can I do X" or "what happens if I do Y", since they genuinely don't know).
This can be remedied by requiring them to read and understand the rules some time. Make them allocate a block of time before the next session to really thoroughly read all the general rules and any specific rules regarding their character (this could cover multiple rulebooks if they use content from several different resources); you need to be available while they're doing this, either in person or via some sort of chat program, to answer any questions they have while they're doing all this reading. Note that the drawback to this approach is that it occasionally produces a rules lawyer rather than a normal educated player.
Edit2: When using a new system, consider using the opposite of the "make the new players read the rulebooks" approach on players you know are rules lawyers: tell them they're not allowed to read all the rules until the group has completed a couple sessions of play. This ensures that they learn & get used to the rules as you have interpreted them along with any house rules, rather than being exposed to the book's rules first. This can slow down the first couple sessions with questions from the lawyers (since they obviously don't know the rules) but in the long run it can really reduce the number of rules arguments along the lines of "That's not what the book says," "I don't care what the book says, I'm the GM and this is what we're doing". (Thanks to wax eagle for this suggestion.)
Another approach is to declare at the beginning of a session that all encounters will proceed very deliberately, with the pertinent rules called out and discussed by everyone along the way. This provides a few advantages:
- It gets everyone grounded in the rules by applying them in context. Reading the rules and applying them in play are two different things, and many players have a tough time making it through rules and truly absorbing them unless there's something at stake.
- It ensures that everyone's understanding of the rules is the same. This is a good way to suss out ambiguous rule definitions, decide on how they'll be played in your game, and so on.
- Everyone will tire of this thorough but slow approach, and it will be much easier after doing this once to convince the players that it is in everyone's interest to learn the rules thoroughly.
Of course only you can know what will work for your group, but sometimes deliberately slowing things down once or twice is a good way of ensuring the pace will speed up thereafter.
As the GM you are in the driver's seat, so take control.
You can create a Quick Reference Sheet (or photocopy relevant pages of the rulebook) for situations that routinely come up in your game (Combat, skill checks, etc). Typically this is a series of tables or definitions condensed into a single sheet or 2.
Also, if your game is going to require the players to do something that you know they are unfamiliar with, then you could help them by having a cheat-sheet of the related rules. Of course I wouldn't give them this until they needed it to maintain the mystery of the adventure.