Where we have found hashtags useful is if we are writing about a trending topic and you want a barrage of extra followers. See what is trending and if you can quickly write a piece about that topic. Then post it a few times, in different ways, with hashtags and watch your follower count go up.
The problem with this being that trending topics tend to be about the latest episode of Glee or the Beckham's new baby so not that relevant to either of our industries! I think we've only once been able to write quickly enough about a trending topic to get any 'juice' from it.
Getting yourself 'listed' is another good method. Find out who your localised tweet listers are. There are a few guys on twitter who have lists dedicated to business in the Bristol area. We follow them, they follow us and, hopefully, we end up on their lists of 'Bristol Tweeters to follow'.
Finally Twitter is great (along with an up-to-date XML sitemap of course) for getting fresh content crawled really quickly. In fact we can get new articles crawled and listed by Google in a couple of hours with a concerted effort (Twitter and Facebook) to get pages noticed.
Hope that helps your Twitter campaign a bit.
