I'd say the thing that I wished I knew most when I started handling journals is, reviewers are fickle.
- Don't expect that if you invite a reviewer, they will accept or decline. Some never respond. My personal guideline is to wait 7 days before concluding they won't respond and therefore invite new reviewers. You might need a different timeline, depending on your field.
- If reviewers accept, it's not a guarantee they will submit a review. My personal guideline is to wait 7 days after the deadline before giving up on the review ever being submitted.
- Per the above, if you want to be confident that a decision can be made on a paper, you'll need multiple reviewers who have agreed to review the manuscript.
- If a paper is taking a long time to process, the most likely person to remind you about it is the author (sadly).
I'd also recommend getting familiar with the journal office. After all, the journal staff is there to help. If you're lucky, you'll have a motivated desk editor who'll do everything possible to make your task easy, e.g. by configuring the editorial management system to suit you (e.g. what is the default time to give reviewers to submit reviews? What is the default reviewer invitation email supposed to look like? etc), reminding you about late papers, answer questions about processing time, potential special issues, and so on. Unfortunately there are some desk editors who'll simply do nothing unless explicitly instructed ... fingers crossed.
- Check your editorial management system to see if it sends automated reminders at a reasonable pace; if it doesn't, configure it to do so because it's a great help.
- If you have trouble using the editorial management system, ask the journal office. If you want to know statistics like "how long is our average time to first decision?", ask the journal office. There's a good chance the required data is stored by the editorial management system.
- Odds are, as an editorial board member, the publisher will be willing to do you small favours. So for example if you invite a review article or publish an article in the journal, the publisher might be willing to offer free open access or at least discount the price. However, you'll have to ask.
Finally, I suggest making some personal guidelines (similar to the two I mentioned above). Because I was handling so many papers, I found it difficult to remain unbiased. Guidelines helped me treat every paper on an equal footing so I didn't find myself waiting optimistically for a late review because I don't have any other reviewers, for example. I know one editor who waited six months for a late review. Poor authors - they continuously asked, and the editor continuously responded with "it's under review, be patient".