Things to include...
1. What you do IF it's relevant to the book.
Obviously, if you're writing a novel about teachers and you're a teacher it's relevant. If you're writing a non-fiction book then your professional credentials will make a difference. Otherwise, your occupation is not relevant.
2. Writing qualifications/writing courses
If you've done an MA in Creative Writing it shows that a) someone thought you were good enough to get on the course in the first place b) you've spent 1-2 years reading, writing, workshopping which has to be a good thing c) that you're used to getting feedback on your work.
Other courses, groups - yes, it shows seriousness about your approach to writing eg you've been part of a critique group for 3 years but generally anyone can sign up to a class, and attending a general class is no sign of ability. Overall I'd be inclined not to list them.
3. Membership of organisations
Yes to organisations such as the Romantic Novelists Association, the Society of Authors, the Society of Children's Book Writers and Illustrators. No to your local writing group unless it's nationally recognised - or you met the agent through it.
4. Publication of short stories/short story competition wins
Yes, especially if on Radio 4 or in nationally recognised magazines. The more obscure the publication, the less impressive the credit. And yes to competition wins, particularly if they're nationally recognised (eg Bridport, Asham, Fish) or plentiful.
5. Publication in newspapers/journals
Yes, if it's in nationally recognised newspapers or journals. Your local parish magazine is not so impressive. Generally, did the journal/paper pay you money? If yes, it's worth including.
6. Your marketability
It may have been a million years ago that I was in Only Fools and Horses, but people are still interested and it's often the first thing that's said about me in the media. It's something in my past that makes me marketable in the present. If you have something similar then mention it.
In the last few years authors have been encouraged to build platforms. This does not involve wood, hammers and nails, but gathering cyber friends and followers through social media such as Twitter, Facebook, MySpace, blogging etc. The idea is that they a) will be a built-in market for your fabulous book when it hits the shelves and b) will have lots of goodwill towards you so they will pass on details of your fab book to all their friends and followers.
7. Something personal
The jury's out on this one. I've read query letters where the personal bit sounded threatening ('I have a black belt in judo') and others where it sounded interesting (a list of quirky jobs the author had had). If in doubt, get a friend to give you feedback on how you are coming across. Or just leave it out.
The general rule here is to include stuff that endorses you as a writer of this particular book. What NOT to include tomorrow...