Demonstrate that you're already excelling at the responsibilities of a Sr. Engineer: You're taking ownership of new projects and crushing it. You're mentoring other engineers. You're making effective product decisions based on dynamic business constraints. Cite examples.
Point to some projects and explain your contributions in terms of customer value. I find that explaining the problems you're solving through a business lens is an effective way to signal that you can quickly grasp new business challenges and run with them. Employers are looking for people they can trust from Day 1.
Hope this helps :)
As for an example resume, I am one of the people tasked with reviewing senior developer resumes at my job and here is my advice: One. Page. Seriously. Keep it to one. You will stand out right away by virtue of that. Beyond that we really only care about your relevant skills and recent experience so put an array of your skills front and center and then follow it with a good amount of detail about your current/last position. The rest you can be pretty sparse on because it only gets a glance.
Best of luck to you.
The reason why the former has had the senior title is down to industry knowledge. They're not the best developer in the room, but they have a mixture of:
* Getting shit done. The quality might not be great, and it might have been late, but they're still there, and they've been at the company while they've delivered something.
* Years in the business. They might not be an expert PHP developer, but they've worked with Drupal for 4 years, and the business is a Drupal shop. This tends to be the common one I see. A senior-level developer in one stack might crumble to pieces in another (source: I was a senior .NET dev, and I'm struggling with Ruby right now).
* Desire to talk. They attend every meetup, and are happy to talk and promote where possible. They've given dozens of talks at meetups and conferences, even though individually they're no better than their peers.
I work with a guy who has about 4 years of experience and is very senior as a Rails engineer. He has a very deep knowledge of Rails, Rails application architecture, and how to manage and maintain Rails systems. Is he senior? Absolutely, at Rails. Would he be Senior in anything else? Nope.
I know another engineer with a Masters and 12+ years of experience. He has a very wide breadth of knowledge but very little deep. Is he senior? Not even close. But he can pick up a lot of technologies and at least get by ok.
Seniority has absolutely zero to do with number of years you’ve been in this business. Find a senior position you want and try to tie your actual existing skills and experience to it.