Know what you are eating. Here’s a couple of tips:
Sugar is bad. Limit it as much as possible. It has many different names on the labels, but the absolute worst one is "high fructose corn syrup". Avoid it at all costs. Also, don’t be a fool and fall for the "sugar in the raw". Just because it "seems" natural doesn’t mean it isn’t a refined carb (by the way, you *need* carbs, just not refined ones such as sugar, white flour (Opt for whole grains instead), etc.)
Bad Fats. You also *need* fat in your diet, just not the bad ones. Partially hydrogenated oils are the worst. Avoid them as an ingredient. Instead, utilize extra virgin olive oil, canola oils, fish, raw nuts (mmmm almonds) and seeds (flax seed is especially good because they contain omega 3 fatty acids and they are a great source of fiber (gotta keep things moving)).
Portion Sizes. Only 1 gram per serving (but there’s 20 servings in a bag). Very common misconception is when someone reads the label and think that they are only getting "x" grams of this or that (per portion), only to find out that they just ate 4 portions. Many food manufacturers pull this trick, so be careful.