SharePoint Designer isn't the best at reporting errors, but there's a few tricks that help with the troubleshooting process.

1. When there are odd errors that make no sense, click on the Information icon see image below.

This error log gives a good breakdown of the workflow errors and shows what's causing the problem.

This is great at identifying the wacky problems that break Designer workflows.

1. Use the Log action in the workflow commands. I sprinkle these throughout the workflow, so I can see the sequence of actions taking place.
2. If you ever have an issue of workflows saving an item multiple times, make sure the Automatically update isn't selected in the workflow settings

1. Similar to point 2. Start and finish with a Log command and use String Builder.

Notice that I'm stating the user who's running the workflow. This is useful with identifying permissions issues of how the workflow is being run.

1. When creating local variables use sense names. See figure below:

With Strings, the variable starts with : str

Numbers: num

Date: d

For a seasoned developer this is obvious, but it doesn't always translate into SharePoint Designer variables.