Preparing for the Viva: After you submit

The most important goal in preparing for the viva is to keep the subject alive in your head.

Try to anticipate the questions you'll be asked in your viva and keep working on a file of anticipated questions (both the generic questions listed on this web-page, and questions specific to particular sections of your thesis) and your answers. If you said anything without understanding it 100%, or anything you have doubts about having justified properly, add it to your viva file.

You can go into university after you've submitted your thesis and your registration has expired - doing some more practical work may (or may not) help to keep the subject alive in your head (you could do experiments and take printouts of the results to the viva).

However, the main preparation for the viva is reading. These are the things to prioritise:

Know your thesis inside-out. Compile a thesis summary (see above) if you didn't do so before you submitted - revising from that rather than from the thesis itself will help you focus on the strategic level (a half-line summary of each paragraph in the thesis should suffice to remind you of every important point in the thesis). You should read your thesis summary in a continuous cycle while you're waiting for the viva, and you should read the thesis itself at least once after you submit it and before the viva. Try to read it from the perspective of the examiners.
If your thesis contains mathematical formulae, check them carefully so that you're confident, by the time of the viva, that they're correct. If they're not correct, work it out in advance so that you're not flustered by mathematical mistakes at the viva.
Be familiar with the references cited in your thesis, because that's the literature your examiners are most likely to ask you about. Read anything you have cited without reading (not that you should cite things without reading in the first place!).
The examiners could also ask you about literature not in the thesis, to test whether you are widely-read in your area.
So make sure you're familiar with the literature - not everything you've read in the last three years, but the more important stuff.
Look for recent review/survey papers of related areas. You need to be able to discuss the state of the art in any area related to your thesis.
Recent publications tend to be particularly important (what are the recent developments in your field?), although they can't ask you about anything published after you submitted your thesis.
Read the examiners' publications to get a feel for where they're coming from, what things they consider important, and which topics they consider relevant.
Don't stop reading until after the viva.

It might be an idea to publish a paper or two between submitting your thesis and the viva - I wish I had done so. Try to write papers from different perspectives.

The time between submitting the thesis and the viva varies greatly. I submitted my thesis on 28th September 2001, and had my viva on 18th September 2002! My thesis was very long (390 pages including appendices), and there was a delay in finding a suitable external examiner, but above all you have to remember that your examiners will be busy with other things too!

The shortest time I've heard of between submission and viva is three weeks (different subject, different university).

They have to give you at least two weeks' notice before the viva. I got five weeks' notice. My internal examiner suggested a couple of dates, I chose 18th September and asked for 14:00 in IT406, and this was officially confirmed a few days later.