I buy everything a season or two prior. In fact I just bought tons of stuff at Old Navy; winter coats for $6, hats & gloves for .99, sweaters & shirts $2.49, puffy vests $4.49 & more stuff. Target I bought 3 pack socks for $1.00, underwear 6 pack $1.49. I usually will find pants for $2.49 at Childrens Place/Carters. But I like Old Navy jeans & try to buy on clearance otherwise I don't mind waiting till they are $10 & using a 10% off coupon or $15 off $75 purchase.
My kids typically will go thru 2 different jean sizes during the fall/winter/spring season, so I buy 2 different sizes of jeans but it works out that I have 2 girls so the one gets the others hand me downs.
I also have been going on websites bookoo.com & craigslist; I found some nearly brand new North Face winter boots for my son. Initially we bought some $65 columbia boots but then found these for $15 so I returned the $65 boots.
I would go up one size for the winter coat & all other clothes as well.
Another thing is that you will typicailly over buy when you buy seasons ahead, but I don't mind because as a child I only got hand me downs from charities & as you know most people donate stuff that looks terrible, which is embarrassing for a kid but I get to donate brand new clothing, coats, shoes that I know will put a smile on a child's face so it's worth it to me.