I like to maintain a clean namespace, which means that generally, instead of recording data on DOM objects, I wrap it in closures. For those circumstances where I absolutely cannot do that (and it is rare), I try to be as local as I can and use a wrapper name...

    DOMObject.my_special_site_id.specific_data1
    DOMObject.my_special_site_id.specific_data2

Nasty global variableses...