One of the problems I've noticed a lot of GMs have when they're in the player seat is that we can be too quiet. Now it's not a problem for everyone - I know several GMs who are very good at being players - but I also know a lot of GMs who basically become wall flowers once made players. Depending on the game, and the size of the group, I tend to fall into this category myself more than I'd like. That said, this post if for all us GM -> Players out there, as well as for the other players who tend to be a bit quiet.
It's Ok To Speak Up
The first, and perhaps hardest, thing to learn if you're normally not the kind to be loud at the game table is that it is ok to speak up. After all, if you don't speak up you can't be heard. All communication at the gaming table is done verbally, and that communication is how you do actions and take care of things in game. Simply put, if you don't speak up you won't be heard. If you can't be heard then you won't take any action. If you aren't taking any action, then what is the point of being in the game?
It's Ok To Interject
I don't mean cut someone off, but when there is a gap in the talking it is ok to slip something in. Odds are that when you are talking, or trying to get the GM's attention, other players will interject with you. Sometimes those things may bring the GM's attention away from you even when you are trying to handle your own stuff. because of this, you shouldn't feel bad if you need to interject to get the GM's attention back on you.
Handle this politely. There is no need to be rude, but there is also nothing rude about getting your GMs attention and asking about the thing you want to handle. If they ask for a minute to take care of what their attention is on, then give it to them. If they go on to something else, remind them that you're waiting. They'll get to you. If not, maybe bring it up after game and see what is going on.
It's Ok To Hold the GM's Attention
Sometimes you are doing something important. Even though you are doing something important, another player is also trying to take care of their important thing. that player asks for the GM's attention, and the GM goes to give it to them. However you are trying to wrap up your thing still and already had the GM's attention. In that case, don't feel bad if you need to ask the GM to let you finish up your thing before he moves on. You may have to specify that it's important to you, but you can do it.
It's Ok To Correct A Player
Sometimes the interjections that another player is giving is because they think you're done, or that their thing may have priority. In these cases it can be ok to ask the player to let you finish your thing instead of the GM. Alternatively, if you are handling something and another player tries to cut in on it, you can let them know that you're already handling it. This comes up most often when an item has been found or multiple people in the group are trying to handle the same thing. Take for example a rare/powerful sword that an enemy dropped. You've picked it up to look at it for some clue. Another player then mentions taking the weapon. Just let them know that you already have the sword. If they want to see it, just like when we were kids they're going to have to wait until you're done with it.
It's Ok To Keep The Cool Thing
Finally, for this post at least, it is perfectly ok to keep the cool thing if you find it, and it is ok to ask for it if you want it. Don't be a dick. Don't horde them all, and don't keep asking for stuff that others may need, but that doesn't mean that you have to give up things because "numerically it's better on X char" or because "Y character saw it first" or even because "Z character asked for it." Items drop, if it is a story item specifically for a character than they should give it, but otherwise if your character uses swords who cares if Sarah or bob have a better fighter. You found the sword, keep it. Let the GM handle keeping the tally even. Long as you're not a dick about it...well, what could you have done wrong?