I do this for all of my kids always have and always will (shoot I even do it for dh & myself!

) Usually I buy one size bigger then what they are currently wearing, unless they are going thru a growth spurt (or haven't in sometime then I figure they are due for one LOL)then I might buy 2 sizes ahead.
Also if the shirts etc. are 100% cotton don't forget to allow for shrinkage just as soon as you don't thinking it's preshrunk or it won't shrink it WILL shrink.
The only thing I don't buy a season ahead is underwear, socks, shoes, etc. I have bought shoes in the next size up if they are something that can be worn anytime of the year like Nike tennis shoes or somesuch.
I figure if none of my kids don't use it (I have a 10 yr. old & 3 yr. old boy and a 7 yr. old girl) that I have plenty of people I can pass it down to, or give as a baby gift if's it's something the youngest one didn't wear at all.
My husband & I were in the Disney Store on Saturday and he mentioned something about a swimsuit for dd for this year....I had to

at him, I bought them months ago! From the Disney store for no more then $5 each I bought several in 2 different sizes so she would have at least 2 for the season.