Normally, we are a Rails shop and very happy with all the tools and community best practices that come out of the box with using Ruby and Rails.
Lately, we've been doing some WordPress development things (just for fun! :/) and the development process just sucked. Even when you put everything in Git deployment and collaboration is still a hassle.
Here are some things that made our work easier:
We've added Compass to our WordPress theme development by using our command line and the compass gem. We wrote a style.css for the WordPress theme info and added our own default stylesheets/ folder where compass would compile our stylesheets to.
Put each theme in a Git
We are working on multiple themes and would like to be able to make changes. We've put the root of each theme in its own separate Git repository. Then on our local development machines we installed WordPress and symlinked our theme repositories into the wp-content/themes of our own WordPress installation.
We can each make our changes independently with Git and also test it in a running WordPress installation.
Use Rake for common tasks
We wrote a few Rake tasks that would help us package and deploy our theme for distribution. Basically, the Rake task compiles the stylesheets, removes development files and adds it all up into a nice .zip file for WordPress distribution.