PCB design checklist

General

A Power LED!
Check connector pin sequence (is it mirrored?)
If it goes into an enclosure, design all connectors to be pcb-mounted
Add holes so that wires can be patched from top to bottom
Check inverted logic is used and named properly
ERC and DRC
Print 1:1 on paper and check new footprints
I2C: pullups ok?
Are decoupling caps at the right place?
Via placement for ground plane?
Check track widths. Enough vias? Via diameter?
No via under pad.
Add test points?
Check thermal management
Silkscreen labels for jumpers and connectors
Verify that silkscreen component labels are next to their component
No silkscreen over pads
Verify mounting holes position
Verify clearance around mounting holes to avoid shorts
Verify 3d clearance if several boards stacked or if it has to fit in an enclosure.
Double-check orientation of connectors
No silkscreen over vias or pads!
Silkscreen board name and revision. Optional which square for tested tick
Machine assembly: add fiducials, check part centroids, check pin 1 location (IPC-7351)
Check solder mask clearance

RF tips

Michael Ossman's rules: 4 layers, ICs, 50 Ohms everywhere, follow recommendations, route RF first

At order time

When using an ATmega: