Q:

How can I show aptitude to prospective employers when my all my work is on internal projects? 

I've been in my current position for a long time (10 years) and in that time, I feel like I've performed well as a designer, system architect, and programmer.   However, all that work has been on internal projects that aren't accessible from the outside world. 
I see a lot of advice like this that suggests 'If you can literally point to something and say "I wrote this" it's very impressive'.   What about if you can 'literally point to' nothing at all, because while you're a passionate programmer who (as the classic Joel-ism puts it) "is smart and gets things done", all those things are invisible? 
Do I need to start frantically committing to open-source projects?   Start a "real world" (not corporate-internal) blog?   Frankly, I spent most of my 10 years happy here, and only recently have considered leaving for greener pastures.   Am I going to be sunk before I start looking because of my focus on work my current employer, at the expense of my "public presence"? 

A:

Showing external projects is helpful but it's never been an blocker for me hiring or getting hired in the past.  
If you can talk about the projects you worked on and explain to whoever is interviewing you some detail about what you did, what went well, how it provided value to your organization.  Getting excited about what you did and programming in general is a good way to score points in a lot of places.  Showing interest in open source stuff, having a github account, even if all you do is follow some projects, maybe a small patch, does show some value.  I've found most employers don't actually try and look at the details of my open source projects on github, they are just excited to see it ;)

A:

It's never too late to start, of course. 
I meet programmers all the time that have no online presence; they're content consumers rather than providers, and yet the good ones never seem to have much trouble finding work.  If you've been in the same place for ten years, hopefully you have a laundry list of accomplishments to list on your resume, and a few people you've worked with that can attest to them.