A basic way of doing this is as follows:
Open skin1/customer/categories.tpl
FIND:
BEFORE ADD (Replace X1 and X2 with your categoryid's to break after, and you can add X3, X4 etc.):
Code:
{if ($subcategories[cat_num].categoryid eq "X1" OR $categories[cat_num].categoryid eq "X1") OR ($subcategories[cat_num].categoryid eq "X2" OR $categories[cat_num].categoryid eq "X2")}------{/if}
A more advanced way to do it, which would require custom coding, is to add a new admin checkmark field "Break After Category" in your admin section, where you could check the box to indicate a line break, and then modify the above code to check the database field instead of manually coding the categoryid.