But, it’s a bit of a hassle and I don’t update the plugin often enough to remember what step to take.
The assumption is that you’ve followed this guide to setup you local repo with svn and git remotes.
Write code in local git feature branches as usual. Push the finished code to Githubs master branch and add a git tag.
Create a new local git branch (or use a clean old one) and merge local master branch (remember to squash all commits!) into this one. It has to have a straight history.
git svn rebase the see that the history matches with the remote Worpress svn repo.
git svn dcommit --username yourusername to push the code to Wordpress svn repo. Remember that you have to change all the tags in your readme.txt and you main plugin file to match the new release version.
git svn tag x.x to copy trunk and create a new remote tag/release from it.