With both cardigans and pullovers I like to switch skeins in the underarm/sideseam area. I could use the end of the row on a cardigan, but I've found it can make picking up for buttonbands more difficult later. Also, if the tension is even a little bit off, it will show at the end of the row (because the end of the row is one of the front edges), while it's much more subtle at the sideseam. I switch skeins on right side rows on cardigans. With a pullover, since you're knitting in the round, you can switch skeins every round. The other thing I do when I alternate skeins is to start alternating partway through the first skein. This way there is more overlap between skeins since I start Skein 2 about halfway through Skein 1, then Skein 3 begins about halfway through Skein 2 and so on. I also hold back a bit from the skeins used in the yoke, divide that in half, and use it to begin the sleeves. Again, it helps with blending the colors. Once you get the hang of alternating skeins and figure out how to manage two at once, it's not hard to do!