Rae mentioned in the comments that yarn substitution is hard for her. And I thought about it a while, and you know, that is kinda hard, in that it's an ongoing learning process, and no one ever gets it exactly right. I don't think I've ever knit a sweater in the suggested yarn (due to being a cheapskate, living in a warm climate, a desire to use up stuff I already have, and a host of other reasons), so I'll throw some thoughts out. Maybe tomorrow we'll get back to more normal blogging - it's been a week of illness (mine and the baby's), broken toes (mine), and migraines around here, so I've been a little neglectful. Bad Samurai.
Anyway. Substituting yarn.
WEIGHT/STITCH GAUGE: This one's a little more flexible than you think. Knitting something at a bigger gauge will produce a bigger sweater -- but that isn't always bad. It WILL change how the pattern knits up, but if you're prepared to deal with that, it's okay. The Yarn Police will not come for you if you deliberately knit a pattern at a different gauge. (I've been known to do it - there's a discussion of how, in my swatching article at Knitty, link in the side bar. Pay attention to my comments on the Dale of Norway sweater.)
FIBER: The easiest thing to do is swap out fiber-for-fiber. You know: if it calls for cotton, use some kind of cotton. But there really are more options than that. Think weight, in terms of how heavy it is per yard. Wool is light; when substituting, use something else light. Cotton is heavy; when substituting, use something heavy. As a general rule of thumb, you can switch around animal fibers for other animal fibers more easily than switching plant for animal or vice-versa. If you do get wild and crazy, here are the general effects: If you knit a pattern for something light (like wool) using something heavy (like cotton), it will hang much lower and stretch out of shape. If you knit a pattern for something heavy (like cotton) using something light (like wool), it will not drape like the original did and often be very short. It is possible to adjust for these differences and get a nice sweater out of the deal. But often it's more bother than it's worth.
COLOR: If it's a solid color, switch out any color you like, remembering that yarn often looks lighter in the ball than it does knit up. If using more than one color, try to identify how the original colors work - do they blend? Do they contrast? - and replicate it with your new choice. Fair Isles often use two groups of blending yarns that contrast with each other. Or one color and a group that contrasts.
VARIEGATED YARNS: Many texture patterns like lace or cables get lost in strong color shifts, though more subtle variations can work wonderfully (a yarn in shades of blue, for instance). If you're trying to keep something from pooling or zigging, think about either slip-stitch patterns, or if possible, patterns with varying numbers of stitches per row. The shifting number of stitches keeps the yarn from settling into a pattern. (For example, shawl patterns where you cast on a few stitches at the point of a V and then increase steadily as you knit will never pool on you - the number of stitches is never constant.)
And here's a quick list of the fibers I can think of offhand, and how they behave when knit up. I'm sure it's not complete.
Wool: light, warm, breatheable. It's got spring to it - if stretched it will resume it's previous shape, at least a couple times.
Alpaca and llama: a little heavier than wool. Very warm. Little stretch or spring - it will drape nicely, but don't expect it to have much give beyond what's built into the knit fabric.
Mohair: very light, very warm, very little spring. Unraveling it is like unraveling barbed wire.
Silk: heavy, no spring, EXTREMELY warm. Warmer than you think. Drapes beautifully, has no memory at all. It can be fragile, depending on the way it's spun. The smooth, reeled silk yarns I adore tend to pill. (The pilling, the cost, and the heat stroke factor is why I don't knit with it much and rarely wear the things I have knit with it. I once wore a silk shell I'd knit to a bar. By the time the night was over it had lengthened by four inches and I was sweating like a pig. Very attractive.)
Cotton: very heavy, no spring, cool, wicks moisture from the body. The usual summer knitting fiber. It can be tough on the hands when you knit with it, but it makes nice sweaters if you don't ask it to do what it can't do - like stretch. It will grow when you wear it, too.
Linen: Not as heavy as cotton, but with even less spring. Also very cool, and wicks moisture, but not as well as cotton. I think it's even better suited to summer than cotton and it's even harder on the hands. You can machine wash it to DEATH, though.
Rayon/Tencel: In knitting it behaves much like cotton. Heavy, no spring. Drapes wonderfully.
Acrylic: light, machine washable, no spring, not terribly warm. High-quality acrylic yarn isn't bad for all-season sweaters, but it's chemically smooth and can slip and slide around when knit, like silk. It's actually quite tricky to work with.
By heavy/light, I'm talking about the weight per yard - that has a lot of impact on what you use for substitution. I'd say it's the most important factor if you want your project to knit up the same way the pattern looks.
By 'spring', I'm talking about fiber memory and it's ability to hold itself up (I'd use the term 'tensile strength' if I were discussing this with the husbeast, Mr. Engineering). Some yarns are so heavy and unspringy that they can't hold themselves up - those are the sweaters that start the night with the hem at your waist, and finish the night halfway to your knees. To some degree the gauge of the yarn affects it (yarn that's just heavier - bulky weight vs. lace weight, say - is of course going to stretch more), but some fibers are just that way. By my experience, silk, cotton, and alpaca are the worst offenders, in that order.
Now if you'll excuse me, I'm going to go hug my kid. (After three days of monstrous behavior and being sick, she is back to her usual adorable self.)