It's in these groups that jobs are often posted before they're posted publicly. And if you've been an active and quality participant in the group, your response to the posting will be looked on favorably. You'll have a foot in the door without even trying. That being said, make sure you are always polite when talking on these groups, as it reflects badly on you if you are condescending, argumentative, or otherwise unpleasant.
When writing up your resume/CV, be 100% honest. Never fudge the truth. Most employers realize that not everyone can know everything. Briefly list the major technologies used at each job, and list a few prominent accomplishments (e.g. "rewrote client-server sync framework, decreasing server load by a factor of 10"). Keep your resume to one page if you have less than 5 years experience, 2 pages for anything over 5. Don't bother with anything other than company name & technologies used for jobs more than 10 years ago. Don't over clutter your resume with skills. Pick your top few. If you start having to write stuff like "XML" as a skill, you know you've gone too far. It's ok if you just have a couple... everyone has to start somewhere.
Never be afraid to say "I don't know" in an interview. If you don't know, say so. No one expects you to know everything, and it's far more valuable for an employer to know that you'll be up front about what you don't know, than to think you'll try to bullshit your way through any question. I guarantee you, they'll know when you're doing it.
Try to work through whatever problems you're given, whether they're brainteasers or coding problems or what "would you do if" questions. Always always talk out loud when doing these problems. It lets the interviewer understand your thinking behind what you're writing. That way, even if you're wrong, the interviewer may see your misunderstanding and help correct it. Most of the time, they're not looking for the right answer, they're looking to see how you work through a problem.
If you're doing coding problems on a whiteboard, always use the language you are most familiar with. It's hard enough coding on a white board without also trying to use some language you've only used in passing. Also, always walk through your code after it's been written to make sure it does what you expect. The interviewer will like that you're double checking your work, and it'll help catch and dumb errors. Think of it as running the code through a compiler and/or tests.
Ask intelligent questions about the problem before you address it... what assumptions are you making? Is it only ASCII or is Unicode accepted? Are we talking 32 bit ints or 64? Does this need to be thread safe? Questions like this, before even writing a single character on the board will show that you are a thoughtful coder that doesn't just blast out the first code that comes into your head.
Finally... dress nicely. You probably don't need a suit. If you're junior, I'd go with slacks, a button down shirt, and a tie. If you're mid level (7+ years), drop the tie. If you're senior (15+ years)... just make sure you dressed in clean clothing :)
Always offer to shake hands when you meet someone and when they leave. Look them in the eye and say "Thank you". Be nice. Smile freely.