Although it isn't possible to prepare for all types of tests (e.g. personality tests), there are a few things that can be done to maximise your chances of success.
Make sure you understand the purpose of psychometric testing.
There is no 'right' and 'wrong' answers in personality tests.
Speed of completion can be a factor, so work as fast and as accurately as you can.
Recognise your strengths and weaknesses! Employers make their selection based on a combination of these. Being strong in one area and weak in another does not necessarily count against you.
Practise, practise, practise! And do so against the clock.
Get a good night's sleep before doing the test.
Keep calm and have a positive attitude!