I personally find (you're free to disagree) LibreOffice's toolbar icons ugly and not quite intuitive, so I decided to replace them... if I only knew how... Any advice? (I'm especially interested in solutions for Windows systems.)
Finally I figured out how this can be made to work. There are some nice icon sets out there, like the one pointed out by the @techfreak in his answer. Unfortunately, unlike one might believe from his rather laconic answer, it is not a mere copy+paste job. The problem is that in newer versions of LibreOffice (3.5+ and that's probably going to happen again sooner or later with other updates too) the directory tree of the icons inside the
zip is altered. So you need to manually compare the downloaded file with a working icon set and once the differences are corrected, comes the copy+paste part, per this post.