For example, suppose you want to space 8 decreases, evenly spaced, on a hat top of 93 stitches. 93 stitches isn't evenly divisible by 8. The nearest even multiple of 8 is 88, which would be 8 decreases spaced 11 stitches apart, with 5 excess stitches left over.
1) Random decreasing before you get to the decrease roundsIn this solution, in the last few rounds before the decrease rounds begin, the 5 stitches would randomly be decreased away by working two stitches together at five random points. By the time you get to the decrease rounds, you would have 88 stitches: 8 markers placed 11 stitches apart. This is the most common solution, I think, and it works very well in garter stitch (use k2togs), reverse stockinette (on the k side, use k2togs, on the purl side, p2togs) and other bumpy fabrics. However, in stockinette, especially in bulky yarns (relatively few stitches) this has the potential to show somewhat as a disturbance in the fabric.
2) Differentially beginning the decrease roundPerhaps better looking in an all-stockinette fabric is this trick: work the first decrease round in pattern, but don't work all the decreases--in this differential beginning to the decrease round, only the excess stitches are decreased away. In our example of 93 stitches and 8 decreases, place your markers in the last round before the decreases as follows: 3 markers 11 stitches apart (light blue on diagram, 33 stitches accounted for) and 5 markers 12 stitches apart (dark blue on diagram, the remaining 60 stitches accounted for). On the first decrease round, decrease only on the 5 markers at 12 stitches apart by k2tog'ing the green stitches--this makes each set of 2 green stitches into 1 stitch, which gets rid of the 5 excess stitches. You would then have 8 markers, each of which is 11 stitches from the next marker, and can go on to decrease evenly on each following decrease round.
|click to enlarge|
|the decreases on this hat top start on a different rows|
I find that this works not only on hat tops (as shown above) but also on raglan decreases, sock gusset decreases and so on. Also, differentially beginning works for increases, also. With this trick under your belt, it is not necessary to cast on evenly divisible multiples for hats, sweaters, etc., freeing you to make garments which fit better.
ADDENDUM, 2014: There is a new TECHknitting post which has more tricks evenly space decreases on an uneven stitch count--the tricks are part of a pattern for a scrap tam.
PS: For hats with a seam, put the excess stitches in the back for slightly greater fullness where it is needed--the (rounder) back of the head, rather than on the (flatter) forehead.
PPS: 8 evenly spaced decreases, worked every other round (one decrease round, followed by a plain round) is the default decrease rate for a hat top. It doesn't always yield a perfectly flat top, however. Switch to smaller needles in the last few rows (as was done in the illustrated hat above) and you'll have more a chance to avoid "knipples" at the hat top.