Parts Cleaning (preferably without poisoning yourself)

When I say parts cleaning, I don't just mean gun parts. I mean any parts that have old dirt, grease or oil (or that disgusting combination thereof) on them.

Some stuff isn't bad, like oil or grease that will mostly wipe off, or dust that can be blown off. But sometimes you can wind up with something that's caked on heavily, or (especially fun) old oil that's dried on a surface, which can be a real problem to remove.

Tools

Depending on how bad the mess is:

• cloths (rags, paper towels)
• brass, wood or plastic for scrapers
• solvents

Basic Level: Wiping
Like is says, this is stuff you can wipe off with a cloth, maybe with a bit of CLP or something to help. Simple and easy.

Intermediate Level: Scraping and Wiping
Here we're talking about oil or grease that's collected a lot of fouling (dirt and other particles, often mixed with dust that blew in) and caked into a disgusting mess on the parts, or the part was dropped into. This is much worse to clean off.

Scraping
If you're cleaning metal parts, use something softer than metal, like plastic or brass, to prevent scratching the material. Those fake credit cards you sometimes get in the mail are good for this. Wood will also work, if the stuff is somewhat soft; craft sticks are good for this because you can cut or sand the end to fit, and even give it somewhat of an edge.
Scrape as much of the crap off as you can, and wipe off the tool with rags or paper towels. If you're using a metal tool on something really stubborn, watch your hands and the angles so you don't accidentally have it slip off the crud and into your hand; considering the gunk, you really don't want it punched through your skin.

If what's left is soft enough, you should be able to wipe the rest off the surface. If it's stubborn, you can wipe it down with a generous amount of CLP and let it sit a while, then wipe it off.

Or you can go to...

Advanced Level: Solvents

DANGER, WILL ROBINSON!

Some solvents are flammable, and quite a few are some level of toxic, which means you must use them in a place with good ventilation, or outdoors, and wear gloves and anything else you think necessary.

Most of these are wonderful at removing stuff, but be careful: in addition to being toxic, some can cause chemical burns to your skin, and they'll remove the oils from your skin just as quickly as they will from the surface of metal. With these, use them either straight, or diluted as directed on the container, and rinse thoroughly when finished, then dry and oil. Be aware that some can damage some finishes, so if you've got some super-duper stuff, try it on a generally out-of-sight spot first.

The two most commonly-used solvents both work very well, and both will mess you up.

• Use low-odor mineral spirits outside if you can, and remember the gloves. It works wonderfully to dissolve the crap that builds up, and when done you can either filter the crud out, or let it settle to the bottom, and pour it back in the bottle to use again.

• Brake cleaner* (also known as carburetor cleaner) also works well. If you do much of this it'll cost more than the mineral spirits, but it has the advantage of coming in a can with a tube that'll let you spray it into tight quarters to flush things out.  This stuff is also good if you've cleaned a part, and you want to flush off any traces of oil before using, say, cold blue on the part. Again, have good ventilation and use gloves.

Both of these are highly flammable, and you don't want any kind of open flame or other ignition source around them, especially when you're spraying brake cleaner.

Something that's often overlooked that can do a pretty good job is dish soap and hot water, especially for a final stage before putting on some kind of finish.

• Wear gloves to  keep it off your hands and to keep your skin oils off the metal. 
• Wash and scrub, rinse with hot water, and dry. 
•  With carbon steel, remember to either oil it immediately or do the finishing, as carbon steel will start rusting fast after this.

Finally, there are commercial degreasers: either like this, often can be found at dollar stores or Wal-Mart), or this.

Nasty Level: Dried-on Crud
I once helped a friend clean her father's old deer rifle, which had been in storage for years. It was a lovely old Marlin lever-action, and you couldn't open the action. No sign of rust; it was just stuck. I worked some stuff in and kept tugging on the lever, and it finally moved a bit, then stuck. Eventually I got the lever and bolt to move, and when I got the thing apart there was not a trace of rust -- but the oil he'd last used on it had dried, hard, and glued the bolt in place. That's the kind of thing I'm talking about.
With something like this, you have to get it as far apart as you can, and then soak it. Anything will do, but I generally go with mineral spirits in a container (with a lid) big enough to submerge the parts.

1. Put them in, cover them with solvent, put the lid on and leave it at least a couple of hours. 
2. Take a stiff brush -- copper or brass works well on steel -- put on your gloves, and scrub and scrape as called for on the parts. 
3. Get off what you can, then back into the solvent for another few hours.
4. Depending on how hard it's dried, it may take a good while, especially out of any pin holes and other such areas. 
5. When I finally get them clean, I like to flush them off with brake cleaner to blow the last of the stuff off, then let them dry, then oil. 
6. Oiling is important: this isn't water and soap, but you're stripping everything off the steel, and it WILL start rusting in short order.

This will generally clean anything you're likely to run across, and doesn't require overly-hazardous chemicals or a chemical suit to protect yourself (although a suitable respirator will be a good idea with some if you do it indoors, or for general safety).

One other thing I'll mention: I haven't used it for this purpose, but you can make a really good penetrating oil by mixing mineral spirits and automatic transmission fluid 50/50; I suspect it'd work pretty well at soaking off dried crud as well. The warnings about skin/breathing protection hold true.


Footnote
* I wait until someplace like Advance Auto Parts has a "buy one, get one free sale"; you get a much better price that way.