Advice for writing a one page resume?

First, I highly recommend checking out our full in-depth 1 hour+ session on writing an effective resume.

To keep things at just 1 page (which can definitely be a challenge as a senior engineer):

• Shrink your education section (I've reduced mine to just 2 lines)
• Reduce or even remove old experience (things >5 years ago)
• Maintain a high bar for skills you claim to be proficient at: A common failure mode I've seen with weaker engineers and their bad resumes is that they'll drop every technology they have ever touched on their resume.

...especially in how to pick apart what's most "important" from my previous work experiences

Here are some signals that something is worth putting on your resume:

• It had high business impact
• It was a major part of your promotion packet (since you're senior, I imagine you've gone through at least 1 promotion)
• It was challenging in some way

For a very detailed breakdown of how to judge the scope (and impressiveness) of a project, check out this Q&A on senior engineer projects.