I maintain a few open source projects, and found this list to align very closely with my own desires: https://davidwalsh.name/45-github-issues-dos-donts

Some highlights:

- Do submit one pull request to address one issue.

- Do submit two pull request to address two issues.

- Do not tack on a minor whitespace or semicolon changes in unrelated commits or Pull Requests, even if the minor change is correct. Make a dedicated commit or Pull Request instead.

In general, the smaller and easier to understand a PR is, the easier to merge. I've seen a lot of PRs lose their way due to a "while I'm here" mentality of fixing code style etc, and it often obscures the real purpose of the contribution.