I started at a size 16/18 in pants, and I'm down to a size 8/10 in pants. Tops I started in L/XL and I'm now in mostly mediums, some large depending on the brand (unfortunately my DD girls didn't go down at all so shirt size didn't change that dramatically). I bought some new pants every size - I had a lot of size 16 and size 14 pants from before I gained the weight, and some size 12. I had to buy more size 12 than 14 or 16, and had to buy all new size 10. I wasn't buying new tops for the longest time because I wasn't quite fitting into mediums yet, but now after 47 pounds I've had to get rid of 85% of my tops because they are ridiculously big.
After all that rambling - I say buying at least new pants for every size - I hated the look of baggy pants, especially at work, plus it makes you feel good to have well fitting pants when you are losing the weight and want to show it. I am also in favor of shopping sales and getting a few good pairs of pants for each size you go down. Tops can be bought less often but still should get new ones when they start looking overly baggy - I know that looking sloppy at work for me won't work.

