Also, if you have headphones on or are talking on your cell phone, please keep all volume to a reasonable level. Not everyone shares your taste in music, nor do they want to hear about your previous evening's escapades, thankyouverymuch.
Pod! Or there's this new thing called "texting"
Plan your trip ahead of time!! Figure out where you are going, where you need to get off, where/when/if you need a transfer ahead of time. Most transit authorities have this info available online and by phone. My transit authority has a service that will walk you through planning your trip by phone.
It's one thing to confirm with a bus driver as you enter, "Is this the 15B or 15C?"; it's another to take up time getting lots of directions from the driver, and even then it's helpful til you've waited til everyone is seated instead of holding up the line because you want to know where to catch the bus you need to transfer to.
Have exact fare ready if you are paying cash, or your card ready if you pay using that. It's not fair to get on the bus and expect that other passengers will have change for a twenty! It's also not fair to everyone waiting behind you on cold/hot days for you to hold up the line looking for money or your card at the bottom of your bag.
If others were at the bus/train stop ahead of you, it's polite to let them enter before you.