Go to your template and select "Edit HTML". Add the following code to the section entitled Posts.
.post pre {
background-color: #efefef;
font-family: 'andale mono','Courier New', Monaco, monospace;
font-size: 1.25em;
padding: 1em 1em;
width: 90%;
border: 2px dotted #ddd;
overflow: auto;
}
Now if you want to include some code in your post, then just do the following.<pre><code>Include code here.</code></pre>
You will have to replace HTML characters (like <>) with the appropriate decimal codes. There are plenty of sites with character tables.