Pick the cheapest cow's-milk based brand you can find, unless your doctor suggests soy-based, lactose-free, or another alternative. Avoid low-iron (under 4 mg per liter) formulas, which the American Academy of Pediatrics says are nutritionally inadequate. (Forget the misconception that iron causes constipation.) The government mandates nutrient levels in formula, so generic choices are fine. And, go for powder over liquid- there's no difference in nutrients or digestibility, and it's cheaper, portable and doesn't need refrigeration.
Whether you choose powdered, concentrated liquid or ready-made formula, always check the expiration date. Be safe about storage: Once you've opened, prepared or refrigerated the formula, use it within 48 hours, and don't leave prepared formula out of the fridge for more than two hours. Toss any formula left in the bottle after a feeding, because germs from baby's saliva will multiply in it. (Gross.)