Tweaking just the .post styles will not be sufficient, as there are many other style parts to your posting:
<div class="post" id="post-34">
<small>August 21st, 2005 by Administrator</small>
Ever had a dream to experience the space flight?
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<p class="postmetadata"> 1 Comment »
You can see from the above snippet from you're site, that in addition to "post", you'll probably have to alter "entry", "postmetadata" and things like "madding", "margins" etc for any other element within the structure to get the look that you want. Basically, just back-up your style.css file and then through trial and error, get the spacing correct for your site.
I'm sure if you woul go ahead and search for things like "Kubrick modification", "CSS tutorials" and what not, you'll find some information that will help you solve your problem.