If you have an alpha-test texture with a black background you get a black border effect around it in-game.
You can see how this is avoided with the terrain asset trees - the edge pixels of leaves are spread outwards.
I used to know a plugin to auto-create this effect in photoshop - does anyone know where I can get one or how it is most easily done?
Here you go 
That method is pretty bad in my opinion. Flaming Pear offers a free plugin called “Solidify” which is much better. I have a YouTube video about it, an there’s a video somewhere on the Blurst site too.
It works just fine for me, but feel free to post links to better methods
Edit: Could just post them myself though. Slaps myself with a fish…
OSX : http://www.flamingpear.com/dl/freebies.dmg
Win : http://www.flamingpear.com/dl/freebies.zip
Looking forward to experiment with those this weekend.
Sorry, it’s hard for me to get links at work because I only have my iPhone here and am not supposed to be using it. 
(Takes more time to get links and format than to type.)