If you are using long wires to transport a signal to e.g. the analog entrances of your arduino, then voltages sometimes can be generated in those wires that are a bit too much for your Arduino and you may blow it up.
A small protection circuit may prevent a lot of problems and aggravation. Of course the Zener and resistor should be placed close to the chip.
Should you be considering building the simple Arduino board, described earlier, then take advantage of the space that is still present on the strip board, right in front of the Analog gates. Cutting the strip, bridging that cut with a resistor and connecting the zener between anolog and ground would be easy to do.
Serial communication with a DIY Arduino remains a topic of interest as the USB connection with an FTDI chip is not always easy to DIY.
A MAX232 as shown before can do the job and so can a few discrete components. But if you have 2 gates left in e.g. a 7404 (a HEX inverter) you can use that one too:
If you have 3 gates left, you could add one to the above circuit to transfer the DTR signal of pin 4 (on a DB9) to use that as a Reset for the DIY Arduino. Just make sure you feed that signal to the input of a spare gate via a voltage divider (2x10K).
Oh well, let me do that for you. It would look like this: