6 Foods (And Drinks) That Aren't Friends With Your Skin...

White Foods
This includes white bread, pasta, cakes and anything else made with white flour. These foods have a high glycemic index, which basically means they cause a spike in blood sugar. Fluctuations in blood sugar lead to excess secretion of insulin and androgens, which causes over-production of sebum, enhanced cell division and aggregation of dead skin cells - all of these are big causes of acne. Whole grains, on the other hand, including wholewheat and oats, release their sugars slowly and are packed full of antioxidants. 
Sugary treats
Like white foods, sugar causes a spike in blood glucose levels (obviously!) Besides increasing sebum production, high blood sugar can weaken the skin by affecting collagen and elastin, which are responsible for keeping the skin firm. Collagen depletion causes fine lines and wrinkles, and can make the skin lose its luster. Basically, any foods that cause a spike in blood sugar level, including sweetened fruit juices, can have a negative effect on your skin over time. So try to balance out with foods high in fibre, which have a lower GI. 
Alcohol
Alcohol is a diuretic, which makes it very dehydrating for the cells, including skin cells. Dehydration causes fine lines and wrinkles in the skin. Alcohol also causes vasodilatation, which can trigger outbreaks rosacea and psoriasis. Also, when you consume lots of alcohol, the liver has to work hard to metabolise it. This means your body is more likely to release toxins through the skin, making the skin look dull and potentially causing breakouts. Caffeine is also a diuretic, so remember to drink lots of water before and after a big night out, and go easy on the coffee the next day! 
Salt
Salt causes fluid retention in the tissue, which basically means the cells swell.  This can have the effect of making your face look puffy. Apparently iodized salt is the worst of the bunch and it has been shown to aggravate acne.

Bad fats, including fried foods and hydrogenated fat
When hydrogenated fats (aka trans-fats) are heated, either when you cook with them (e.g. frying in vegetable oil), or during their production (e.g. margarine and ghee), they become oxidised, which produces free radicals. Free radicals cause damage to all cells in the body and they disrupt the skin's structure by destroying collagen and elastin. Besides damaging the skin, they have strong links to heart disease, cancer and diabetes, because they disturb the DNA structure at a cellular level. Besides obviously keeping consumption of trans-fats to a minimum, I recommend cooking with coconut oil, which oxidises at a higher temperature. This makes it less damaging to our cells than other oils.
Dairy (Maybe...)
I'm not sure how much evidence there is to support a link between milk and acne, but certainly people have said anecdotally that dairy products (most notably cow's milk) causes them to break out or that since they've stopped drinking milk their acne has cleared up. Apparently cow's milk contains certain hormones which overstimulate the glands that produce the skin's oily secretions. Goat's milk, and even buffalo milk, apparently contain less of the harmful hormones. So they might make a good substitute. 

...Eating these foods won't necessarily directly cause breakouts, but consuming them regularly or in excess will stop your skin from looking its best. At the most basic level, a diet high in vitamins, minerals and good fats will make you look healthier than a diet made up of empty calories and lacking in nutrients. It might be a bit unrealistic to say you're never going to eat fries again, but just make sure to balance it out once in a while with foods high in antioxidants, like different coloured vegetables, to fight off those nasty free radicals. This will keep your immune system strong, your cells healthier and, all in all, your body will function better. Thus making you look all glowy and stuff :)

(Images from stock.xchng)