I would have basically said whats listed above. Just make sure your PCT is solid and your recovery is efficient. In a sense you can't prevent acne because you are putting so many things into your body and then the estrogen and/or androgen simply causes acne in most people. There are many things that you can take to help prevent acne but I am not gonna list them cause IMO they are a waste of money. Sure you may get some spots for a few weeks but it isn't too bad. I do promo work so I can't be having spots so I do care about the subject. Certain vitamins are good for your skin but I find when you are taking so many substances (putting things into your body that are not natural to it - or the levels you are taking aren't) then the less is the best. Basically meaning when you throw in a multi-vitamin that is often the cause of more spots cause your body goes through other changes.
The reply from drob covers most but other tip for you would be drinking lots of water (that is key). You should be doing that anyway. The best tip would be to buy white tea (or green tea). Just google it. You should be able to buy it in certain supermarkets and all herbal shops (mainly asian). Green is good but white tea contains more antioxidants then any other tea. Green/White tea is also good for cutting you up too (just a little add on won't be taking pounds off you). Antioxidants are good for your overall well-being too and can be found in many things. The best are berries (blueberries, raspberries, cranberries, Blackberries, Strawberries etc). Nut are really good for your skin too and for your diet (bulking)... try almonds, brazil nuts, walnuts etc etc.
Combine a great PCT schduele, washing/showering regularly, certain lotions, drinking lots of water and white/green tea and eating lots of the above and I guarantee you will feel twice as good as you usually do. You should just be doing the above cycle or no cycle. So my suggestions will help prevent them from the inside and make you feel better and look healthier in general and the other suggestions are brilliant for using on the outside to help prevent acne and get rid of the acne you do get. Hope it helps.