Make a list before visiting the supermarket. List according to priority e.g. put yogurt above that pack of chips.
Read the labels- While buying packaged foods; always read nutrition labels and ingredients along with the expiry dates. This will ensure that you get freshest products which will last longer.
Avoid food-like substances- Yes, there are certain "foods" which are not really food but a product of the processing industries. Mostly things with too many ingredients which you can't recognize (e.g. HFCS-High fructose corn syrup) are the ones to avoid.
Go organic- If you want the best for your family then don’t skimp on buying good foodstuffs. Also by doing this, you will not only keep your family healthy , but also help those few people who have taken care that no pesticides should pollute the food.
Visit the farmer's market regularly- the quintessential "sabzi mandi" is the closest we come with regards to local produce and seasonal veggies. The freshest produce with reasonable price tag- Definitely the choice to make.
Finish the items on list before impulse buying- The things you need the least are either at the end or not present in the list.