I love buying clothes on clearance!!
I buy one size bigger than the age he will be for that season. For example, summer stuff - he will be about 18 months next summer, so I'd get 24 month size. But I guess it depends on how your child is. Your little girl seems like she's probably in 12 month clothes, so adding one size up from age sounds about right.
For stuff that isn't season dependent (plain short sleeved onesies, normal weight pants, socks) I'll buy a few sizes (24 month, 2T, 3T etc)
For things that I don't want to buy and then possibly not have him wear - infant snow suit, baby swim trunks, shorts, it depends on the price. If it's a crazy good deal, I just buy one. If it doesn't get worn, I'll give it my my sister for my nephew that's like 10 months younger.
The whole size thing is much much easier when they are older. Last spring I bought SS a winter coat and snow pants, he was wearing size 3T last year, so I bought him 4T on clearance. Buying something bigger is always a good bet, cuz they just keep growing and it will fit them eventually!