This first one is a no brainer, but when I play in a game I like to sit somewhere near the GM unless I know the environment is going to be quiet. Why? Because the closer I am to the GM, the more likely I am going to be able to hear what he/she says, and thus the more likely that I will be able to react in a timely manner.
This is more an issue for games run in a store, or in a house where half the people present aren't playing in the game, but outside noise can be distracting. When it is constant - again, think if you're in a store - and you are further from the GM, it can actually drown out the sound of the voice you want to hear and make it harder to keep up with the action. Really, it is just better to be closer to the GM some times.
Cross Talk Can Be Bad
On the player only side of things, I've found that it is better to put players whose characters interact a lot next to each other, rather than across from each other. Honestly, the less people that a straight line between these two can cut off, the better. Why? Because talking through an active conversation is hard. I know there have been several times in games I've run where I've either had to, or wanted to, ask people to end their IC conversation because I - as the GM - couldn't hear what the player I was talking to was saying. I don't like doing this, especially when it is IC conversation, but when player 3 is doing something that could spell the fate of the game, and players 1 and 2 are just kind of bantering, I - as the GM - need to hear player 3. For this reason, if no other, if you can get players whose characters are close to sit close to each other, it works out all the better. This way, when they're having their little IC pow-wow (which is a GOOD thing) it isn't cutting off any other conversations from happening.
Mentoring A New Player
When you have a new player joining the game, where you have them sit can have a huge impact on their game experience. Personally, I like to sit them next to someone who knows the mechanics of the game very well, and is also an active player. This does two things. One, it gives the new player someone near by that they can ask rules questions to. This means that they're not constantly asking me, the GM, these questions which makes it a whole lot more unobtrusive for the game itself. Essentially, the flow of the game doesn't change, because the newbie has someone to talk to. Two, it puts them near an active player which gives them a front row seat to see how things are done. Often, this player will also take steps to help involve the new person, getting them into the game. Again, this works because their mentor is right next to them, and they have someone they can quietly talk to.
So far, this has worked out very well for me in my games. Often, a lot better than having the person who doesn't know the game sitting next to the GM. Yes, that means the GM can help them out, but it also means that whenever the person has a question, the GM has to shift focus to that player. This can actually hinder the person's ability to learn, as they quickly notice that when they have a question the play - and the fun - stops until they are informed.
Not Playing The Game
The last thing, I generally don't like people who aren't in the game to be sitting at the table amidst the players. If there is room at the end of the table, sure, whatever, but I don't want someone not in the game in between me and any player not in the game. This may sound rude, but my reason for it is simple. The person isn't in the game, and so if they are sitting amongst the players they are a distraction. Every move they take, every thing they say, is going to distract someone from what is going on in the game at that moment. If they are sitting at the end of the group, they are likely only going to distract those two people. If they are sitting in the middle of the group, they are going to distract everyone in the game.
Now, I will freely admit that I'm a bit of a dick when it comes to distractions at the game table. If someone in the game wants to heckle, that's fine. Someone outside of the game? No, I don't like it. I take my gaming fairly seriously, and people who break the group dynamic just bother me. This is why the first, and only real, rule that I have when someone asks if someone can watch a game is: can they be quiet and stay out of the way? If the answer is no, then they are not welcome. If they prove to be loud and not quiet while I am GMing, I will ask them to leave. Rude? Maybe. But that 4-5 hour time frame when we are having game is something the group scheduled to do together. As the GM, it is my responsibility to make sure that that time is able to be used for what we want. This is also why I warn people wanting to watch a game, that watching a table top session can be one of the most boring things you'll ever do in your life.
Still, if you are having someone join your game to watch a session, don't be afraid to sit them just outside of the group (the affect not doing this can have on group dynamics is insane) and just ask them to be quiet. If they look confused or lost, or there is a quiet moment, just call a break and talk to them. But during game, try to keep the focus on the game.