A guide to more finished PCBs

If you ever had your PCB manufactured at factory, odds are you're familiar with the following situation. You design a schematic, lay out the board, check it multiple times, then send Gerbers to manufacturer and some time later the box of shiny PCBs has arrived. You assemble it (or don't, some manufacturers offer a full assembly at moderate cost) only to find that something SO obvious is amiss.
There's so much to keep in mind when designing a schematic and PCB, and it's even harder when you don't do it regularly. That's why I decided to compile a list of

things to check before you submit your PCB to fab

Schematic

PCBs start with schematic diagram, that's why it's important to tidy it up first.

1) Draw dots at junctions. How's your schematic capture sofware supposed to know these crossing lines connect and those crossing lines do not? That's right, with the help of dots.

Well-arranged multivibrator. Notice the dots at the crossing junctions.

2) Add filtering capacitors: big electrolytics for low frequencies, small ceramics for high frequencies.

Two ceramic capacitors by the part for bypassing.

Small caps (100 nF generally) should be placed as close as possible to IC pins (on schematic, too!).
In serious analog circuitry also add inductor/mutual inductor/inductor-capacitor filters for better noice reduction.
Good links to learn about power supply filtering:
Designers guide manual on power supply filtering

TI SCAA048
Analog MT-101
Maxim AN4713
Intersil AN1325

Filter calculator

3) Add protection - fuses, flyback diodes etc. (a thin trace can happen to be a fuse - see item 1 under "PCB")

NPN transistor protected by flywheel diode	Ditto for PNP

4) Add some means to program and test your rig!
Any fresh board needs testing and programming first. That's why you need a way to make temporary connections for measuring equipment. For these purposes you have a choice between the following:
Test pads for pogo pins

Some guy's great and educational solution

TagConnect ICSP Cable

Great choice for cases when you need maximal portability and high-density PCBs.

Test points for hook clips

Сonvenient in most through-hole designs.

Regular pin headers

(I assume everyone knows how those look)

Well, if you haven't stocked up on real test accesories, it's your choice.

PCB

1) Check the thickness of every trace. Insufficient thickness of any trace will result in that trace burning after you connect power to your PCB. Look at my example of PCB that modulates flyback transformer's arc so it would play music (nevermind the horrible arrangement of PCB):

I've outlined the path where high currents are switched through the primary of flyback xformer.	Aaand it corresponds to 2 traces that are only good for carrying signal!

Now this looks better:

Power traces should be thick even if they don't carry much current, it makes them more recognizable on the board.
2) 45 degree rule
It doesn't matter how you route your schematic if the signals don't go higher than 1MHz. Still, routing your board with 45 degrees everywhere makes its appearance more aesthetic:

Arbitrary angles	45 deg. everywhere

3) Arrange names on silkscreen, add meaningful designations wherever possible. Imagine you see your board for the first time and you have to figure out what to do to make it work - what to connect here and which jumper to close etc. Example of PCB for a wireless network node can be seen below:

I didn't take heed at proper silkscreen when I ordered the first batch which resulted in ugly boards	This looks better. Names, dates, pin names - everything is there.

4) Add ground and power planes for shortest possible current return path length. Also add planes under surface-mount components that tend to heat when used (voltage regulators, FETs etc.) for better thermal performance.

That's obviously not everything you have to keep in mind. All I described in this article is based on personal experience I have aquired over years of doing PCBs as a hobby.