You can block in various ways, Knitty.com has some great tutorials, but I like to full on wet block, or wash my knitting. Fiber content will also factor in, but i find most anything can be washed if you're careful. In addition to blocking and shaping, I like my sweaters to actually be clean.
So, since we're now on our 3rd snow day, I thought I'd do a visual blocking tutorial:
You will need lukewarm water, baby shampoo, 2 sets of towels and a basin large enough to submerge the piece you're washing without smooshing it.
You might also want "help"
Once you have filled the basin add the shampoo (or wool wash like Euclan, I like baby shampoo because it's cheap and readily available and it's designed to clean something made of protein) between 1-3 teaspoons, depending on the size of what you're washing/blocking and froth the water to disperse the soap throughout. Drop in your knitting.
Now gently press out the water - don't wring, just press against the side of the basin, turn the work and repeat.
Now lay out your first towel flat on a counter or other waterproof surface and lay out you piece on it. Doesn't need to be shaped, just laid flat in a single layer.
Now roll up the towel into a big cigar:
Now press firmly all along the roll the remove water. The towel will wick away water and pad you pressing to keep the piece from stretching. The towel is also insurance against agitation, which can cause felting/pilling. When you unroll the piece you'll see the absorbed water in the towel:
Finally lay the piece out in a warm dry place, shaping gently for a garment, using wires or pins for something like a shawl: