I hate Walmart for a host of reasons and, although I know many swear by its low cost options, I try to avoid it like the plague. I have found the best place to get the most for your dollar is the thrift stores in your area. Not all thrift stores are created equal. Here's a breakdown of what I've found.
Salvation Army Thrift Stores
The clothes are tagged in here with different colors based on what day they came into the store. Each day of the week there is half off one of the color tags. Unless I'm in search of something I need immediately, I try to shop by tag color. Also before major holidays (Christmas, Memorial Day, Easter, Labor Day, etc) they mark multiple tag colors down to clear things out. We often head to the cabin for the holidays and hit the store close to home and the one we pass on our way.
I find these to be a little more pricey. But they follow a similar discount each day pattern.
Hospital Thrift Stores
These are smaller and hit or miss. The prices tend to be higher and the discounts fewer.
My plan of attack is to generally have my eyes open all year for the needs of the upcoming year. I got a ton of clothes for school back in May at the Memorial Day sale at the Salvation Army Thrift Store. I got a ton of summer clothes around Labor Day & Columbus Day when they were discounted. See the pattern?
I try to hit the thrift stores about every two weeks, sometimes I buy nothing sometimes I hit the jackpot.
Tag sales are also great and usually cheaper than thrift stores. Friends and relatives who buy like crazy are a great resource as well. Your local Freecycle group is another great place to check for clothes. Check out freecycle.org to find a group near you.