I already have had this kind of problem. Here the code I used
for(var i = 0;i < formName.selectBoxName.length;i++){
if ( formName.selectBoxName.options[i].value == sessionVariableValue){
formName.selectBoxName.selectedIndex = i;
break;
}
}
I don't know how you get the session variable but this code should help a little bit.
But if you don't set any value to the options of the selectBox, replace in the code formName.selectBoxName.options[i].value by formName.selectBoxName.options[i].text.
For information, options of a selectbox have 2 principal properties: text, which is displayed in the page, and value, which is sent to the url set in the property "action" of the form.