The other day I reviewed the book Very Easy Circular Knits by Betty Barnden. In the review I said it was frustrating trying to convert a flat pattern to one knitted in the round. Betty doesn't cover how to do this, but I have a method that I use.
When converting flat knitting to circular knitting it is important to remember that the pattern is written to accommodate sewing a seam. So usually an extra stitch or two is added to the front and the back.
First, I determine how many stitches I need to cast on. This is usually done by adding the number of stitches needed for both the front and the back (which is typically double the front or back number) minus two stitches.
Taking the extra stitches out is especially important when using a stitch pattern that is knitted in multiples of stitches. If the extra stitches aren't removed then the stitch pattern will be off.
Take for example a pattern that states:
K4, *P3, K3* until end of row, P3
The extra K at the beginning needs to be eliminated so that the pattern is knit in the round with K3P3
After I cast on the stitches I've calculated that I need, I knit in the round until I get to the armhole shaping. Then I put half the stitches on a stitch holder. I consider these stitches the front.
I then knit the back, shaping the arms and the neck according to the pattern. I then do the same for the front.
I also knit the sleeves according to the directions. Then all I have to do is seam the armholes and the shoulder seams. Much better then having to seam the sides too!
You can knit the sleeves in the round to avoid seams there as well but at the cap you need to knit back and forth.
Now if you want to do the whole sweater in the round without any seaming then you might try a top-down raglan. There is a great top-down raglan pattern generator at Knitting Fool.
I've knitted a sweater using one generated from this site. The nice thing about top-down sweaters is that you can try on the sweater as you go and not guess if it will be long enough!