I always buy ahead for my kids. When they were babies, I got tons of stuff at garage sales since there were sooo many baby clothes for next to nothing. It seemed like finding clothes in 3T and up in decent condition for a decent price at garage sales was pretty hard, so I've been shopping a lot of sales at Old Navy, Sears, TCP, etc the past couple years. TCP outlet usually marks everything down to $3 and then $1 or $2 at the end of the season. I got a lot of cute things for dd a couple years ago that way.
I scored at Old Navy a couple weeks ago. They had an additional 1/2 off all clearance, and I also used a $10 off $50 coupon (from the website or oldnavyweekly.com) and got clothes for the kids for garage sale prices or just barely above. I got lots of shirts for $0.50 (Christmas ones) to $2 each. I got fleece pants for $1.50 or $2. And the best deal are the winter coats that were normally $40+ that I got for about $4 each!
I also buy shoes ahead for the kids. Payless often has some great clearance deals. If I find a shoe I like but they don't have the kids' size, I have the clerk look up the size I need in their computer. Sometimes a local store will have that size, sometimes they don't. If a local store does have it, Payless prints out a slip with the store location plus a coupon for $3 off that pair of shoes! So if I found a $6 pair of sneakers and a store 10 minutes away has it, I pick it up there for $3 with the coupon!