Inserting list element in next to last position

Inserting list element in next to last position

Given a HTML unordered list
<ul id = "master_key_list">
<li id="master_key_23" class="available_element">Fix the Peanut Butter</li>
<li id="master_key_24" class="available_element">Focus on Price
Sensitivity</li>
<li id="master_key_25" class="available_element" >Focus on Messaging</li>
<li id="master_key_26" class="available_element">Create Growth and
Retention Funnels</li>
<li id="new_bp_element_element" class="available_element">Additional
Best Practice?</li>
</ul>
how can I add another list element before the last list element?
I've tried
$("#master_key_list li:last").before("<li />").append(appendString);
where appendString is a valid concatenated HTML text. However, all this is
doing is overwriting the last list element text, rather than actually
inserting it.