How should I clean a part before installing it in a vacuum system? What are proven procedures for preparing a part that comes fresh out of the workshop for ultra-high vacuum (UHV) and extremely-high vacuum (XUV)? 
 A: I find in this link of the Beams department at CERN, how to clean parts of a quadrupole because it will be in a vacuum.

Cleaning of the parts
Since a quadrupole is working under vacuum and high currents are used
it is essential that they are absolutely clean from any oxides, grease or other contaminants.
It is also of high importance that the pole tips are completely smooth;
because of this every part should be absolutely clean.
To clean the parts one can first use gasoline together with a special tissue to rub all
parts respectively and then followed up with alcohol to degrease the surfaces.
During cleaning (and assembly)it is very easy to get unwanted grease on
various parts;one should therefore use plastic gloves and make it as a habit to change them very frequently
.
Although this cleaning is good for a start it is not enough so to have every part
completely pure one should contact a service here at CERN that remove oxides and
perform a deeper cleaning by washing the parts in acid baths

This was the first hit searching the Beam site.
A: What kind of vacuum setup is it? In our lab we dealt with a bell jar style and cleaned it out with acetone. As long as you're not rubbing it onto rubber pieces, it works great.
A: our lab has an ultra-high vacuum stm system (10-11 torr), and all parts that go in the vacuum system has to be extremely clean. Here is what we do:
first i want to point out that the material you use for UHV is very important too. The commonly accepted material is 316 stainless steel and oxygen free pure copper. For other specialized material, you should check before applying it in the vacuum system.
for any piece that just came out of the workshop, clean it with a towel to remove the visible grease/dirt. Don't worry about the stuff inside the threads or hard to reach places yet.
If you have a sonicator (sonic bath), then you can sonicate your piece following this list of solvents, for 20 mins each:


*

*detergent (we use sparkleen)

*acetone (skip when cleaning copper)

*ethanol

*methanol


you will need clean gloves and work bench (layered with aluminum foil).
The point to use a sonicator is to remove all the gunk in the hard-to-reach places, like the inside of threads. You can't use just acetone, since while it is great at removing grease, it sticks onto your surface and thus would contaminate the UHV environment. So you need the lower molecular weight solvents to get the acetone off.
If you DON'T have a sonic bath (I recommend you get one if you are doing serious work with UHV), you are able to get away with the cleaning by electrocleaning (we don't do electrocleaning with UHV, and I don't know if it will work 100%). The process with electro cleaning is available online. Use the solvent list first, to get rid of the surface contaminants, and use cathodic electrocleaning followed by anodic electrocleaning. The electrolytes can be found commercially. (we have an cleaning agent called Tivaclean from Krohn)
A: Note that for UHV it is recommended to vacuum bake the component ~250C. Please make sure the component is bake-able first.
A: For vacuum applications what matters is the outgassing rate of any object brought in, and also the inner surface of the vacuum vessel. So cleaning depends on what the contamination is, but the recipe in the answer from @cokeheaven is a good one (I skip the methanol step)
What I missed in the other answers is electropolishing. You can have your metal pieces electropolished for little money from commercial suppliers. At the expense of a few micrometers of material thickness, that greatly reduces the surface area and thus the outgassing rate. Once it comes back from the company do follow @cokeheaven's cleaning recipe though, as electropolishing uses electrolytes that may have who-knows-what in them.
Finally, consider getting a Residual Gas Analyzer (RGA). These run for a few k$ and tell you precisely which contaminants are in your vacuum, so from that you can improve your cleaning procedures if needed. I have one permanently installed on all my systems (and use them as a leak checker at the same time)
