My experiences with PCB design:
The vendors I have used accepts 1mm text hight as the minimum. Others can do even smaller but its getting more and more difficult to read.
For small text sizes: Use 0.1 mm apperture. Keeping the relation "apperture/text hight" to ~0.1 will provide good readability.
Compress the texts, minimizing distance between characters and by making text hight > text width.
I always design polarized thru-hole component (copper) pads with a square for positive (electrolythics), cathode (diodes), DIL-foot prints with a square for pin #1.
If ordering prototype boards without silk this eases populating the board.
Place reference designators outside the foot print and closest to Pin #1 if possible.
Do not allow silk screen inside (underneath) any multi-pin foot print. Many PCB packages have "auto symbol creation" but they almost always create and place the silk in the wrong fashion.
Once populated, silk inside sometimes then hides pin #1 indicator.
When creating a footprint: Design the silk screen (outline) so that it does not cross any pad. Some PCB packages can automatically remove such silk screen elements
but IMHO it is better to do this already at the fp design stage.
And as others have suggested: For test points - add test point signal descriptions in the silk screen.
And finally: turn off all layers except silkscreen, copper pads/pins and vias. Check that no silk sits above (SMD)/pins or vias and that all texts are oriented in the same way, readable from left to right or from bottom to top (or the other way around if preferred).
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