What makes something 'good' is always going to be subjective, but when it comes to university admissions you need to demonstrate why you have chosen medicine, how you have prepared yourself for the demands of the course and what else you can offer the university.
Why - this is always going to be tricky! Think carefully about what it is that either first caught your attention to medicine (although it is difficult to avoid being cliche), how a medical degree is different to a bioscience or nursing degree, or any experiences you have had that have led you to the feel medicine is for you. Give any evidence you can to back up assertions e.g. I have chosen medicine because it is a caring career and I am a caring person. One example of this aspect of my nature is shown by my commitment to volunteer work - both at the local charity shop and care home.
How - of course work experience is very important to show you have an understanding of a career in medicine, and know that it isn't quite as glamorous as Casualty might lead you to believe! If you've done any work in hospitals or GP surgeries that's brilliant, but universities know that it can be tricky to get this, especially if you don't have any friends or family who work in such places. So volunteer work in any capacity to do with children (think Brownies or breakfast clubs), the elderly (care homes, even buying shopping for your next door neighbour) or the sick (not all caring happens in hospitals). Equally you should write about any particular books you've read that have inspired you, any lectures you've been to or if you've been on a summer course etc.
What - this should be the shortest part, maybe only 20% of your PS. Here you include all extra-curricular activities you participate in and, importantly, how they have given you skills that will make you a good doctor. Anything can be justified for example teamwork skills that come with hockey, or personal drive and time management that comes with competitive swimming. If you've run or been a member of a club or society, or simply enjoy doing something casually in your free time, try to include it here to show that you aren't just focused on getting the grades - after all, a doctor has to interact with patients as well as try to cure them.