To me PUGs represent everything that a "Massively MULTIPLAYER Online rpg" is. It's about other people. Meeting other people, getting to know them. Interacting with people. Working with what you get.

Only sticking with your guild or worse ONLY ever soloing is really pointless if you think about it.

I used to hate pugs until I changed my attitude on it slightly.
1) Be extremely polite, but don't put up with people's crap. If somebody is talking trash, engaging in bad behavior (rolling need when they can't use the item/pulling before the tank is read etc), warn them once, boot them the second time and get somebody else.

2) Talk to other people in the PUG. Joke about stuff. Ask them about their gear/character/news whatever. Obviously you don't have to blabber on constantly, especially not when the group needs to focus on fights, but whatever you do, engage with people. Don't just sit there and treat the other players like NPCs.

3) Be polite. Use full sentences and words where you can. People tend to respect you more and have more confidence in you when you type like a mature human being.

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Quick tip:
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This may be obvious to some, but I figure a few newbies out there don't know about it. When you need say just 1 or 2 additional party members and there is nobody in your LFG, there is another way to get somebody.

Work out the level range of the instance. Let's say you're doing Drak'Tharon Keep. It's around L74-78.

In the chatbox type: /who 74-78

This will give you a list of all players on the server in that level bracket. You can then start selecting classes that would be a good fit for your group and POLITELY, I mean ULTRA POLITELY ask them if they would like to accompany your group. Tell them you already have say 3 or 5 members and what you need.

DO NOT just spam invites. Look at people's locations. If they're already in an instance, do not bother them. Explain what you need them for clearly.

e.g. talking to a shaman you would like to heal for you: "Hi there, sorry for bothering you, but would you be interested in healing with us? We already have 3 DPS and a tank."

If they decline, send them a whisper saying thanks for the reply and wish them well.

Very often you can find the last few members for a group in minutes this way and you'll likely make some new friends along the way.