I've yet to find any good intro-level PCB design references. There's plenty of good information out there, but it's generally not easily accessible, and is scattered around blogs, forum posts, open hardware sourcefile dumps, etc. There are some fairly decent videos from the likes of Dave Jones (of EEVBlog), and some of the Mikeselectricstuff(.co.uk) that talk and sometimes show the interesting parts of PCB design.

What's lacking (or I've yet to find) is a simple tutorial-type progression that explains both how, and why, to do certain things, especially when it comes to things like component selection, footprint design, part layout, and actual track routing.

The "try it, make mistakes, learn" approach is almost inevitable, but the cost/time of the feedback loop can be pretty steep for a hobby (you can have 24-hr turnaround on boards if you've the $$$, or you can pay $5/board in 6-8 weeks, but the middle ground is tricky and messy. Plus all teh different board houses might have different design rules and other things that complicate matters for beginners.

My suggestion would be to find some Open Hardware kits or designs, buy a couple to make them, but also get the design files (hopefully in either kicad or eagle, which are free/affordable, and then start by ripping up all the routing, and seeing if you can re-track all the components get get it to pass ERC/DRC checks. Maybe bounce it off a few people in /r/askelectronics or eevblog forums, and then have it sent out for fab. If it works, pick a new OHW project with harder things, and repeat :)

That saves you from having to deal with footprint, schematic capture, and BoM decisions initially, and focus just on teh board design parts. You'll want to get there eventually, and you can learn a lot from how other things are put together.

Maybe pick some simple circuits (basic CMOY headphone amp, say), and redraw the schematic in your EDA tool. Then gather up teh datasheets, and usually they'll have a recommended footprint drawing, and you can learn how to build those in your toolchain.

There are some open libraries of component footprints (I think sparkfun has one? Maybe octopart as well?) but it's a useful skill to learn, and easiest on little things with few pins before you work up to the 192 pin FPGA or something :)

* Always print your designs 1:1 scale before sending out for processing.

* Always check your design file outputs (gerbers) in an external viewer to make sure they look ok/make sense.

* Remember to flip top/bottom layers the right number of times! This should happen properly on export, but it's easy to screw up and have the perfect board if you can find components with the exact opposite pinouts :)

* Don't be scared of starting with surface-mount. You can usually get things in pretty big packages if you want, and an 1812 or 1210 is actually not far off the size of a normal resistor body (minus the leads). You can go smaller as you're comfortable and pick up technique.

* I can't really offer any layout tips, because I'm still not very good. If anyone has pointers to good docs on:
(a) how to decide where to place/orient your components on the board to begin with, (b) choosing placement/routing grid sizes and track widths/via sizing, (c) appropriate layer count/usage - when to add more layers, value of pwr/gnd planes, use of fills, etc. And lastly, with kicad especially, how to fix things when you've routed yourself into a corner with just a few pins left, but no way from A-B (preferably without either adding layers, or through-hole links. I know about the 0R resistor bridge, but it has its limitations.