How do I sharpen photos on my phone?
I have been using PicSay's free version and been happy with it.  I'll probably upgrade to the full version which has the ability to sharpen, but just haven't really had the need yet so I've been putting it off.  I'll report back if I do chip down.  

Another alternative would be to use Pixlr's web based editor: http://pixlr.com/editor  

Provided you have flash installed you can use their browser based editor that does have an "Unsharp Mask" filter.