Ok, as the title dictates, this should be another common sense thing, but as it isn't so common, here you go. This one's for the newbie priests out there (and some not-so newbie who still do it the other way). Again, all flames will be | to /dev/null, so don't bother =)
This isn't so much of a guide, as a little trick.
When soloing, I have noticed a bunch of priests pull something first, then shield up, do some dps, then start to get beaten on by mobs. Why not cast shield again right away? Correct. Weakened soul, a 30 sec. debuff which does not allow you to be shielded again until it's gone. This is where the trick lies.
This is not needed for all mobs. Some you can take down without much of a problem the traditional way, but for some of the tricky pulls, or PVP encounters where there's a good chance you could die, try this.
Cast shield on yourself and wait for the pull until you have around 8 seconds left on the debuff. As the shield lasts 30 seconds, or until you have taken X damage (based on lvl), you'll have time to dps away on the mob/player while under the shield, but also be able to immediately re-shield yourself if you take too much damage early on. It doesn't seem like much, but those 8 or so seconds you saved from waiting can be the difference between a /dance or a long walk from spirit healer with head hung in /shame.
Again, this is geared more towards lowbie priests who haven't got the hang of it yet. Hope this post helps someone out there =)