Write clear meaningful git commit messages (Do read this).
Make sure your PR's description contains GitHub's special keyword references that automatically close the related issue when the PR is merged. (Check this for more info)
When you make very very minor changes to a PR of yours (like for example fixing a text in button, 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. (Learn how to squash at here)
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.