The Viva itself

The PhD viva is an open-book exam: you can bring any materials you want. Here is what I think one should bring to the viva:

a copy of your thesis, obviously - you can stick yellow `post-it' notes on it (e.g. anticipated questions and answers), although I personally abhorred the idea of preparing from my thesis itself;
your list of anticipated viva questions and your answers;
printouts of the results of any post-submission experiments;
the chapter-summaries you made for revision;
all the notebooks you should have been keeping since the start of your research (the notebooks need to be indexed so that you can look things up);
any papers such that when you reviewed them in the thesis, you regurgitated something they said blindly without really understanding it (in my case, I identified two such papers, but I brought a dozen potentially contentious papers to the viva);
printouts of any files or emails containing useful ideas which you haven't documented elsewhere;
tissues, paracetemol, &c. in case of any unexpected bouts of sneezing, headaches, &c.

At my viva, I gave a presentation (using slides) about some experiments I did after I submitted my thesis. But it's unusual for the candidate to give a presentation, and your supervisor should advise you if it is appropriate to do this. If you do give a presentation, be prepared to be flexible - I was asked to speed up and just give the highlights.

It is not the norm, in this department (I do not speak for other departments/universities), to be expected to give a practical demonstration of your work at the viva, but you could always offer to do so if you think it will help your cause (unlikely).

Anyone can attend a PhD viva, but only the examiners and the candidate can participate. (This means it may be a good idea to attend someone else's viva before your own, though I've never had the balls to gate-crash a viva! :-o )

Your supervisor should definitely attend your viva, although (s)he is usually not allowed to participate unless invited to do so by the examiners. It might be an idea to keep an eye on the body-language of your supervisor to see if you're going wrong! ;-)

A viva typically lasts two hours (but as long as it takes - mine lasted 2h22m), and a common approach is for the examiners to go through the thesis sequentially, asking questions.

Just because they ask a lot of questions doesn't mean you're going to fail. They don't give away the result before or during the viva, but you may be asked to wait around for the result at the end (about half an hour), so that they can explain the result to you - particularly if you have to resubmit your thesis (failure without the option of resubmission is very rare, and is not going to happen if you submit anything resembling a sensible thesis).

Tips:

Relax and enjoy it, if possible!
Ideas should flow out from you without a lot of prompting.
Listen carefully to the questions and take your time answering them.
Answer your questions succinctly (a rough guideline is 2 to 3 minutes each - no 20-minute diatribes!). Avoid going off at a tangent.
Try to make your answers initially inclusive (spot overlaps), analytical, and then if appropriate dismissive or point out the limitations - and the effects of these limits.
Generic viva questions, such as the ones given in the section below, require imagination to answer well!
Answers may utilise a wide variety of examples and domains. They are a test of your breadth of knowledge as well as depth of knowledge which is expected of a PhD student.
Handling difficult questions:
If you don't understand the question, ask for clarification. Paraphrase the question in your own words and say, "is this what you mean?" State your assumptions.
Treat vague questions as invitations to tell the examiners that you know your area and how it fits into related areas. Try to link the question to the questions you have anticipated and their stock answers. After writing a thesis, you should have one big, connected network of discussion in your head, so you need to jump in at the appropriate place for a given question.
If they have a misconception about your work, try to pin it down and explain it.
If you think the question is irrelevant, explain why you think it is irrelevant (it may be that you need to be more broad-minded).
If you really can't answer a question:
Be honest.
If you have any idea at all, say it.
Say, "I can't answer this on the spot, but I should be able to work it out in my own time."
If it's about literature you haven't come across, thank the questionner and ask for a reference.