Knitting sweaters is such a crap shoot, there are four things to consider (let’s assume your knitting technique is up to snuff for the chosen pattern). You have yarn, pattern, fit, and a what not to wear-ish should I even bother or is it going to look like ass no matter what I do. Keeping this in mind has frequently made it very difficult for me to decide on projects. Since although I mentally see myself as a frail Rowan type…Uhh I’m not. In fact I’ve got shoulders a football player would be proud of. I should point out here that my take on knitting garments is that they should be something I would be willing to buy if I could find them. I try to avoid that special sort of ugly that only crafters seem capable of. You all know what I mean, the sweaters that are so ugly that the inner snarky bit goes WTF? So in the spirit of making hand knits vastly less painful to the viewer and much more flattering to the wearer let’s take each in turn and examine them.
Yarn:
Yarn is so many different parts to a project. It can be color, texture, drape. It can emphasize any mistakes or hide them because of the texture of the yarn. I tend to prefer smooth yarns without much texture but that just means any mistakes are glaringly obvious.

(an example from my very early knitting)
You can start with the yarn and find a pattern to fit it or find a pattern you love and go from there. I find for me I half decide on a range of yarns and then when I look at patterns I sort of refine it until I have a good fit. Another thing to keep in mind are colors. What you can get away with in sock yarn you can’t necessarily get away with in a sweater. So its helpful to know not only what colors you like but what colors look good on you. Like I’m dead white and have rather faux red/burgundy hair.

For those of you who have no idea what I look like. Nice bathroom shot eh?
Yellow is so not going to work here. In fact most pastels just make me look washed out and zombie like. I do much better with saturated jewel tones. Well and black, but I have this thing about knitting in black. I wear black every day but knitting in it seems like a waste. Skrilla posted some scans from color me beautiful that you might find helpful to take a gander at. I think most people know what colors will work for them but sometimes in the midst of mad yarn lust it’s easy to forget that this is something you are going to *wear* and no matter how much you like that color, it quite possibly might make you look like cat puke. For example most knitters I know fall deeply and hopelessly in love with hand dyed yarns and/or variegated colorways. We’ve all seen the examples of when it works

and when its a hopeless mess.

Now these are the exact same pattern but one works and the other doesn’t. Now imagine that splashed across your chest! It’s a vision of horror isn’t it? Friends don’t let friends knit ugly.
Moving on to texture…Fun fur is evil. If you want to look like a dead muppet go ahead and make that fun fur sweater. Ok more seriously there are times and places for novelty yarns but usually sweaters aren’t one of them. There are some lovely ribbon yarns. I have never knit with them so I can’t show you any examples but I’m sure for a summer knit they work up into lovely garments.
Also it’s incredibly important to know what the yarn can and can not do. I found the knitters book of yarn to be helpful in this regard. Like ribbing in alpaca…strictly decorative but a nice wool/alpaca blend can handle it. When I made Wicked for my mom I did it in a cotton, the cross over cable pattern on the neckline was totally wasted so I did a plain ribbing that worked much better.
In the end all you can do is swatch and make sure you like it. Although I am queen of a square inch swatch….I usually do test pattern elements just to be sure. Knitting is supposed to be fun and how awful is it when you finish something and then go What the Hell was I thinking?