This little essay assumes that you are living in a city, you use the bus, the tram, or the metro rather frequently and you have to go many stations with this way of transportation, because it is useful no matter the conditions are.
I am living in a big city (the capital of Romania), I go to college every day (almost) and I have to cover great distances, at a rush hour. I always use the bus, it's alot faster (not as fast as the metro, but... here's my reason --->) and alot cheaper - in fact, it's free, I never use a ticket, so I don't pay. Oh well, I am a very lazy person and I am always seeking for comfort. So when I'm in the bus, I am always trying to get to sit down, whatever the conditions. And the result is that I am always getting to occupy a seat, even if the bus is packed and full of people. Here are my techniques. I developed them in years, it takes practice and patience, but in the end you get what you want: sitting down, looking at the window, admiring nice chicks while in the bus people are fighting for space. (mean, huh? :-))))

Tips and tricks:

1. Never sit in neutral places, like near doors, in the middle of the bus etc. Try to get as close as you can to the seats. And I mean CLOSE. Never let another person between you and your target, so that when the one that occupies the seat gets up, you can immediatly conquer the seat.
2. Never choose a target that is close to old people, pregnant women, sick persons. The common sense requires that you should let them sit. Try to find a place surrounded by young or middle aged, healthy looking people.
3. Always keep your eyes peeled at the little "signs" that someone wants to get off the seat and get down the bus. That is:
   • looking trough the window as he/she tries to find the station where to get down
   • closing the book (or the newspaper) he/she was reading
   • arranging the bags (grip them tighter) or the perce
   • asking other passangers about where is the station
   Anyway, you get the picture. More hints can only be found by yourself, after careful studying.
4. Always try to occupy strathegically the places in the bus near two or more (three, four) seats. Those "groups" of seats are controlled by one specific position. Stay in the middle, as for one passanger gets up, you can quickly take his/hers place. If you are controlling more seats, the more probability will be that you will be sitting sooner. Never "control" one person seats, it is a waste of time, unless you are forced to, or you've seen some "signs", as described above.
5. Always be careful at the stations with high degree of interest for the passangers. That is: downtown, central, near malls, near universities, theathers blah blah. Many people get off the bus there, and you should carefully guess who (check below).
6. Try to know your passangers. Just by analysing them, how they look, hearing some conversations between them etc, you can learn valuable things. For example, if you know that there will be a mall in the way, and you see one of those perfumed, make-up-ed, hair died, elegant looking chicks, you might guess that she may go shopping (and in most cases she is) and get off the bus the next station. Go next to her seat! Another examples: passanger: student - station: university ; passanger: worker - station: factory. You get my point.
7. Once you've occupied the seat, never give it away foolishly. You should always give the seat to pregant women, sick persons, elders. Never give the seat to mummies or grannies with sons or nephews that can stand on their own two feet. (It happened to me once. I got up to give the seat away to some granny, and she put her spoiled 8 year old bastard on the seat, while she kept standing up. Bleah!)
8. When desperate, and none of the tips above work, there is one solution that never fails. However, it is an emergency solution, (that is, when you are VERY tired) and should not be abbused. Well, when nothing works, try to look sick, very sick. Go to the nearest seat occupied by an young or middle aged person and ask him for the seat, motivating that you are feeling very ill, or you've had surgical operation recently, or you suffer from back disease, whatever. If you pick your "subject" well, he should give away the seat to YOU. Satisfied? DON'T SHOW IT! Act ill, until you get off the bus.