a. Advantages and Disadvantages
In general, a wash is a layer of transparent color painted over a base color scheme. The wash tends to accumulate in deep areas, creases, and pits; and so the transparent color accumulates and therefore looks darker in these dark areas. This effect simulates shadows, or body hair, or dirt, and can effectively highlight protruding areas.
You can simulate the effect of a wash using pre-colored varnish from the hardware store; this is referred to as "The Minwax Dip Method" because of Minwax's very convenient line of "Polyshades" colored varnishes available. Expert miniature painters consider this to be cheating, and you will not win awards with the Minwax treatment; but if you're just trying to make something look nice on the table for game night, then who cares? Minwax polyurethane varnish provides the added benefit that it is an extremely durable finish, and will protect your paint job (and miniature) better than spray lacquer alone.
b. Washes
In theory, you can create a wash by taking some dark paint, then adding a paint additive known as a 'flow reducer' to it (try this on the palette, not in a bottle), which should cause the paint particles to drop out of their suspension, and therefore settle in creases on the model.
In practice, I have never gotten this to work well with paint. I used a dark dark red to create a wash for a monster, but the wash didn't settle too well, and it just looked like the monster was covered with rivulets of dark gore. It's an interesting look (creepy as heck!), and I just decided to go with it rather than repaint the whole thing, but it isn't really what I was hoping for. Adding a bad wash at the end could mean that you need to repaint the whole model all over again.
You can also buy ready-made washes, but they're hard to find. I don't have experience with these.
Many experts do something similar 'by hand' and without flow reducer; they dilute some dark paint tremendously with water (again, on the palette) and only paint a very light amount (almost dry-brush it) into the creased areas which they want to darken. Then they let that dry, and repeat the process several times. Each individual application is nearly invisible, but the color slowly accumulates. This can be very effective, but it is also very time-consuming, painting those areas over and over and over again.
c. Dipping
Once your players defeat the giant spider, they are not going to be so thrilled fighting an identical giant spider; you will want refreshing new monsters for the next session, and that means more miniatures. I admire the talented artists who create stunningly realistic color and shade effects on the tiny models we have; but I also run the game, and I have lots of jobs to do in order to prepare for game day. I use the Minwax dip method a lot, and it helps me finish lots and lots of miniatures in my very limited spare time. I realize it's a cheap shortcut, but it's a sacrifice in quality and craftsmanship I'm willing to make at this point in my life.
I mentioned before that the popular choice for this is the Minwax Polyshades line. Polyshades is especially convenient because it is available in small half-pint cans (no need to buy the big quart can). I will eventually want the surface of the miniature to have a completely flat non-shiny surface, so I use Satin finish varnish rather than Gloss; with the thin layer I paint on, it usually winds up shiny anyway. The color recommended most often for this is "Antique Walnut," which adds a brown color that works very well with skin tones. But for robots, subterranean monsters, and numerous undead creatures, I use the dark gray color provided by "Tudor;" be forewarned that Tudor can drain a lot of vividness from your paint scheme. Do not use "Black," because it's like dipping your miniature in tar; no light nor color will escape.
I put my miniatures on little wooden disc bases; the disc is sized to the character's "personal space" for combat purposes, and I think the wood material looks a little classier than a black plastic base. I used to get these laser-cut discs from Gale Force Nine, but Gale Foce Nine stopped selling them, and now I have to get them from craft suppliers. Fortunately, 1-inch, 2-inch, 3-inch, and 1/2-inch laser-cut plywood discs are not too difficult to find from craft suppliers. I bring this up now because it makes sense to attach the wooden disc before you varnish the miniature; the varnish enhances the appearance of the wood, as you might expect (the varnish is intended for wooden surfaces, after all). I gently but firmly pop the miniature off the pedestal. I attach the miniature to the wooden disc base using cyanoacrylate glue (I discussed cyanoacrylate glue in "Assembling" above). Then I glue the miniature's new wooden disc base back onto the pedestal with the Duco cement (also discussed in "Assembling").
Dipping is where that working pedestal really comes in handy, because you do not want to touch the wet varnish, even if you're wearing painting gloves. The pedestal also eliminates the concern of where to put your miniature while the varnish dries: on the pedestal, the varnish will not stick the miniature to another surface, nor will the varnish pool and leave a "wafer" of dry varnish attached to your base; the pedestal keeps the miniature suspended, so the miniature dries virtually in mid-air.
Although they call it "dipping," most people who do the "Minwax dip" do not actually submerge their miniature in varnish. Instead, they brush the varnish on. Me, I usually submerge the miniature and brush the excess varnish off. Again, it's all about the time savings.
Minwax polyurethane varnish is oil-based, and very difficult to clean out of brushes, even if you use the toxic and volatile solvents required. I use another shortcut with the brushes when dipping: I don't even try to clean it out of the brush. For the dip stage, I use extremely cheap kids' watercolor-style synthetic-bristle plasic-handle paintbrushes, and when I'm done putting the Minwax on a batch of miniatures, I throw the cheap brush away rather than trying to clean it. Yes, it's wasteful, but comparing the time required to clean out the brush, considering the toxic and flammable solvents, and the limited benefits of cleaning out the brush (that brush is never going to be as good as when you pulled it out of the box), I don't feel so bad tossing out a twenty-five-cent plastic brush; if I use that brush to varnish 12 miniatures in a batch, the cost of that brush amortizes down to about two cents per miniature.