I've added a brand new (hopefully working this time) item stacker. Unfortunately it doesn't yet work for those of you who are using Internet Explorer as your web browser, but I'm working on it .
Just tick the "arrange mode" checkbox in the lower right corner when you're dressing up your pet. Then you can click on items on your pet and click the up and down arrows that appear to move the item below or above the items that are sitting on top of it.
This is all brand-new stuff so please let me know if it works well for you.