Tips & tricks for increasing open-rates and click-rates?

Its all about timing and content in my experience.  I try to write emails and subject lines that speak to the person reading it.  Is it helpful?  Is it timely? Is it relevant to that reader?  Too many businesses abuse their access to a persons inbox or text messages and it is making customers harder and harder to communicate with.  I try to respect the access I have been given and in turn my open rates are typically pretty good. 

For Seasonal Services  
The highest open rates are seasonal booking reminders, seasonal cut off dates for booking, pre-season discounts, emails regarding how to request work online to receive priority service during the busy season

All Businesses 
Periodic check ins, introduction to client hub, service reminders, thank yous, and new product launches

Content
-Short and to the point messages about things that matter to the person I am sending it to.
-If a graphic is added to the email, using alt text in case it doesn't load and having any important information from the image listed elsewhere in text with a clickable link.

Timing and Frequency
I treat email marketing as a slow burn form of marketing I don't expect every email to be a hot seller I want the emails to plant a seed of who to call, how to book, why to book, and when to book.   

I try to establish trends and reach out to customers before they reach out to book.  Internally it keeps the phone from exploding on a single day and customers find it convenient because it is one less thing to worry about.