Best way to create icons (buttons) in Finder toolbar matching OSX feel?
It seems that there remains no official support to add your own buttons to the Finder, despite applications like Dropbox implementing this. The hack is possible (but very complicated) and some instructions can be found here: How to write OS X Finder plugin.

Instead I settled for a screenshot (cmd-shift-4) of one of the existing buttons and edited it into my own icon using an image manipulation program such as GIMP and convert it to a .icns file. The downside is that this means that the button is static and has no animation when it is clicked, and when the window is out of focus things look a bit weird. The icons have to be square.



Here is the template image that I made for anyone else who wants to create their own icons:



and they can be assigned to an app by overwriting /Contents/Resources/applet.icns

Drag the app to the toolbar (holding cmd-alt for Mavericks) and then reboot to make the new icon visible.