XHTML strict:Adding break between line tags

I have a list and i would like to add a space between the <li> items.

Has to be XHTML strict, so we cant use a <br /> tag.

Example:
<ul>
<li>item one</li>
space
<li>item two</li>
space
<li>item three</li>
</ul>

Any ideas?

In CSS.
ul li {
margin-bottom:15px;
}

That worked! Thank you.