If you would like something pinned this is my guidline. I'm not sure what the other admins use, but this is mine.
1. When is the event? If it's more than 2 weeks away I won't pin it - unless it's an item like the WCC
2. Does it really need to be pinned? Will pinning really matter? I ask that for a couple reasons because people miss the pinned items frequently. I tested this by posting the same info in a pinned topic and in an unpinned topic. The unpinned topic had 5-6 times more action than the pinned topic. You can bump unpinned topics to keep them near the top if need be.
3. Don't yell or tell us to pin it, ask please.
4. How much crap is already pinned? If there is very much then it's unlikely to get pinned. Too many pinned topics make the page cluttered feeling.
Have a good day!