Write clear meaningful git commit messages
Make sure your PR's description contains GitHub's special keyword references that automatically close the related issue when the PR is merged.
When you make very very minor changes to a PR of yours (like for example fixing a failing travis build or some small style corrections or minor changes requested by reviewers) make sure you squash your commits afterwards so that you don't have an absurd number of commits for a very small fix.
When you're submitting a PR for a UI-related issue, it would be really awesome if you add a screenshot of your change or a link to a deployment where it can be tested out along with your PR. It makes it very easy for the reviewers and you'll also get reviews quicker.
When you file a feature request or when you are submitting a bug report to the issue tracker, make sure you add steps to reproduce it. Especially if that bug is some weird/rare one.
To join development, please set up the project on your local machine, run it and go through the application completely. Press on any button you can find and see where it leads to. Explore.
If you would like to work on an issue, drop in a comment at the issue. If it is already assigned to someone, but there is no sign of any work being done, please ask first so that the issue can be assigned to you if the previous assignee has dropped it entirely.