In terms of personality assessment there is no way to ‘prepare’. As organisations and positions are different there is no ‘perfect’ profile and as many personality questionnaires contained sophisticated lie detectors or validity scales, any attempt to skew the results is likely to be of detriment to the outcome. When completing personality assessment, it is very important to be honest and answer accurately. It is also essential that you listen carefully to the test instructions that are provided to you and thoroughly read any preparation materials that you are sent prior to commencing the questionnaires.
In terms of abilities assessment this is more difficult to answer. One cannot prepare for the specific content of an assessment, however there may be some benefit in practicing technique. For many it may have been years since they last sat an assessment of this type, so re-familiarising oneself with question types and working under time constraints may assist to improve confidence leading into the assessment. There are a variety of free practice questions available on the internet, including those contained on the PSI website.