Reducing electrical noise

- You can use alu foil, but make sure it touches the metal part of the USB plug.
- Remove the metal support from the USB extension cable on the end where you connect the RTL stick. This will eliminate noise picked up by the USB extension cable.
 - If you have a spare car battery, connect the metal part of USB plug to the negative terminal.
- Mobile operation or no car battery? Wrap the center conductor of coax around the metal part of the USB plug, then place the stick in the middle of coiled coax.

Testing setup

Your antenna receives noise and signal. Without an antenna any signal received is by the RTL stick and the cables, and you do not want that.
For this test: Nooelec RTL-SDR 820T stick, max gain, no antenna. Weak local FM radio station signal 5/5, clear and enjoyable. That is unwanted signal entering the reception chain.
On the images the middle signal is the radio station - that should not be there, smaller peak is better.
Also note the waterfall display for received signal strength.

Results

Direct vs USB cable connection: USB cable (76cm) increased noise by 13 dB.
USB plug metal connected to 1m coax: coax cable center conductor connected to the metal support of the USB plug. Decreased noise by 7db.
USB plug metal connected to 10m wire: further noise reduction by 5dB.
USB plug metal connected to 12V car battery negative (-) terminal: noise almost completely eliminated.
USB plug support connected to 10m coax, RTL stick nesting in the middle: noise completely eliminated.
Aluminum foil, metal cans, metal enclosures: aluminum foil NOT touching either the antenna jack or USB connector, half roll of aluminum foil: no effect. Metal from Seven 0.5 l cans wrapped on top of the alu foil: no effect. Metal enclosure: no effect. Note that if the alu foil/metal/enclosure is connected to the metal part of the USB plug, immediate noise reduction of 15-20 dB, station still heard, speech distinguishable.

USB connector mod

A reduction of 10 dB, stick still safely connected.
Remove the USB metal part that keeps the RTL stick in place,  from the end of the USB extension lead where you connect the RTL stick. 
Only USB signal and power connectors remain. Mow  shielding in the cable is not connected to the signal chain.
Do not remove the other end of the USB extension lead, the one going into the computer USB port, no further reduction realized, but the extension cable will easily come out of the port.

Organize cables at home

If it has a plug or battery, it will radiate electrical noise, either the cable / power lead or the actual device itself.
A simple solution is to place cables and extension leads into a computer case or a similar metal enclosure.
In the following example, unmodified original RTL stick, no antenna so it only picks up noise, connected with 1.24m (5 foot) USB extension cable. Stick resting on the edge of the PC case where I keep cables/extension leads.
PC case panel off: Local station washed away in electrical noise, audibly louder noise.
PC case panel on: Relief from the loud noise, local station understandable.
If I'm after a distant signal: laptop unplugged, electricity turned off in the house, candles on. Optionally: LNA and shortwave upconverter running off batteries.
Best is no electricity: listen to a far-flung Caribbean station in candlelight with your significant other.

Ferrites

Ferrites are rings, beads or clip-on pieces of iron used to reduce noise.
Coil the USB cable around a ferrite ring, or use clamp-on ferrites.
Use ferrites at the terminations of your
- USB cable between computer and stick,
-  on the cable between stick and antenna.

6-7 dB reduction of noise visible, audible decrease of the FM station with 4 turns on a ferrite ring and ferrite beads on the USB cable (USB plug metal removed both ends).
Also did experiments with turns around ferrite rings - 8 turns made no difference.
Conclusion: invest in clamp-on ferrites, the bigger and more, the better.

Maximum noise reduction

Stick wrapped in alu foil, foil touching the metal part of the USB plug, connected by an USB extension cable with the metal removed. Stick inside a metal enclosure, in the middle of 10m coax coil.
Result: no signal whatsoever, FM band is clear, waterfall is uniform blue.
Optional: ferrites if you have them.

Closing thoughts

If you chase very weak signals, such as shortwave with an upconverter, or a weather satellite just on your horizon you need the best noise reduction solution.
In all of the above tests I had gain to the maximum, so any reduction is clearly visible.
Noise as called here (same thing, complicated words):  electromagnetic interference (EMI) or radio frequency interference (RFI).
You know a better option? Curious to hear from you, use the comment section here.


Note on Nooelec: As their customer support might recommend the hardware guide on this blog, Nooelec sent a free RTL-SDR stick to my former high school physics lab.
I like this attitude.