There are three ways to select your seat in a live game:
(1) When you are brought to a table with more than one open seat, you can choose where to sit.
(2) When you sit down at a table that is just starting, you can choose *when* and where to sit. Meaning: you can wait to see where some of the other players sit first before choosing your seat.
(3) At any point, you can request a seat change button and change seats with someone who leaves.
In all cases, the principle of where you want to sit is the same: You want the worst fish with the biggest stacks sitting directly to your right. It's really that simple. Having position is a big advantage and that advantage is most beneficial against someone who plays far too many hands and holds on to them far too long postflop. Moreover, the more chips a bad player has, the more chips that are likely to make their way to you.
If there are one or more fish who are really short, I don't care if they are on my left or not. The reason is that position is most important for turn and river play, so if their stacks are shallow enough that you can put them to commitment decisions on the flop or turn, their positional advantage is neutered. So when you are selecting a seat, the most important factor is where the deep stacks are sitting and how good they are.
Here's an example of how valuable position is against a poor player who is deep. The final board reads K T 5 - 2 - 7 and you hold A J. You cbet the flop for 1/2 pot and were called by the fish. You continued with a 3/5 pot bet on the turn and were called by the fish. What do you on the river? If you are out of position, you have to decide between betting small hoping to get a call from a Kx or maybe even Tx hand, betting big to get called by AK, two pair and small flushes, or to check hoping he bets a flush and you can check/raise. What if you are in position? Well if he checks to you, you might reasonably assume that his range of hands that made a flush is less likely, since he may have bet out with them, so you might make a medium sized bet because think his range isn't strong enough to call a huge bet. But if he leads into you, he probably has a flush, and you can use that information to decide how large a raise he will call. While this dream scenario does not come up too often, being able to extract several hundred additional bbs when it does happen is huge for your winrate.
Finally, a logistics comment. You'll often get asked why you changed your seat, especially if you have just won a few pots in that seat. (The most common reason other players move is because their seat is "cold". Some do this before asking for a setup, others try the setup first. Either way, it's both quite annoying while simultaneously lolworthy.) Here are a few things I've come up with if I can't complain about being card dead: (1) I want a better angle to watch TV / check out some girl. (2) I can't see the board from my original seat (works good if you are at either end of the table). (3) It was too tight over there (people too close to you). (4) As a last resort, I complain about the smell / breath / demeanor / conversational ineptitude of one of the people next to me. Given that it's live poker, reason (4) is going to be applicable about 80% of the time, but I try to bring it up unless I can't come up with anything else remotely plausible. But the most important thing is to make sure you don't reveal your real reason, both because you don't want other regs trying to do the same and you don't want to upset regular fish who might start to stand up if you sit at their table because they take offense at you targeting them.