Well try not to wee yourself for a start..not in the interview room anyway...
Do you have a job description? If so you should be able to work out a lot of the questions. Have a look at the person spec, where it says what skills/abilities/experience etc the person needs.
You can almost guarantee there will be at least one question related to each of the things listed. Those things are listed because the successful candidate must have them, so the purpose of the questions really is to give the candidate a chance to demonstrate that. Other questions may pop up as well obviously, but it gives you a good framework to start with.
So look at all the things you need to have from the list on the person spec, and think of an example of when in the past you have used x skill or demonstrated x ability. Including background, why the situation was successful, what you learnt from it, whether you would do anything differently etc.
If you've got those examples ready in your head to cover all the things on the person spec, you ought to be able to handle most questions. Talking about a past example rather than a hypothetical 'I would do x, y z' is more convincing and reassuring for an employer and is easier for you as well, you will feel more confident talking about stuff that's happened than you would trying to imagine what you would do in x circumstance.
Even if the questions asked are hypothetical 'what would you do' ones, make sure you answer with examples anyway. Also if you have some examples worked up in your head you will feel more confident as you won't go in completely blank about what you are going to say for the next hour.
Other than that, hold head high, relax your shoulders and neck and keep relaxing them - that's where you find yourself automatically tensing up. Just every so often make yourself aware of your shoulders and neck and consciously relax them - it makes a lot of difference.
Good luck!