I've been making mittens from an inspiring book called Magnificent Mittens and Socks by Anna Zilboorg. She makes really colorful mittens, a lot of which have long gauntlet-like, flared cuffs. The purpose of these cuffs is to make the mittens big and colorful enough that they won't get lost as easily, and also, in my view, to keep snow from going up the sleeves of your jacket when you plunge your hands into the powder to make a snowball to throw at your friend.
But the flared cuffs, although traditional in parts of Scandinavia, look pretty girlie to me, and I'm making mittens for guys right now. So I just make a regular ribbed cuff, or sometimes I just narrow the wrist area by switching to smaller needles.
Anna Zilboorg also uses what she calls "the invisible thumb." This is a thumb that sort of blends into the pattern of the palm. It's not exactly invisible, but it does merge nicely with the palm pattern, as you can see in my mitten below:
Anna Zilboorg's strategy for making this thumb is to stop knitting the mitten when you get to the point where you want to insert the thumb. You put the main mitten aside, and knit a thumb from the top down, and then you graft it onto the main mitten at the thumb place by knitting the back-of-the-thumb stitches together with some of the palm stitches.
But this is not how I do it. I do it the way I learned from Elizabeth Zimmerman: the thumb trick, as she calls it. Some call it the afterthought thumb. The truly "afterthought" way to do the afterthought thumb (or afterthought pocket, or afterthought heel) is as follows: you snip a few stitches in the middle of where you want to put the thumb, pocket, or heel; you carefully unravel the stitches for as wide a section as you need to insert the afterthought thing; you pick up stitches on either side of this gap; and then you start knitting the thumb or whatever. (Darn the ravelled ends in later.)
The great thing about this method is that you can knit "blanks," that is, tubes of some length which can then become a sock or a mitten or whatever is needed at the time. You don't have to know when you're knitting the tube whom it will be for; you can decide that later, and at that point you will know how much space to allow between the end of the hand or foot and the thumb or heel. I imagine that many knitters in the past just knitted blanks, kept them in a drawer, and when somebody needed a sock or mitten, they pulled out a blank, snipped a thread, and in a few minutes a new sock or mitten was had.
Since I know who I am knitting these mittens for, though, I knit the afterthought thumb with some forethought: I insert a thread at the point where the thumb should go (the EZ thumb trick). When I come to that point, I knit the stitches off onto a different colored thread. Usually for a thumb this might be about two inches worth of stitches. Then I slip those stitches back onto the left needle and continue knitting around, as if nothing had happened. Here's how it looks:
Later, when the hand of the mitten is finished, I pull out this contrasting thread (in this case the blue thread) and pick up the stitches above and below it. Then I knit the thumb from the bottom up.
Here you can see the thumb stitches on their needles ready to be knit up:
I usually start on these two needles and then add two more, as it's easier to knit circularly on three or four needles. I pick up a stitch in the two gaps at either end of the mitten opening, to avoid a hole there.
Another thing that's distinctive about Anna Zilboorg's mitten method is that she knits them from the top down. To do this, you use a figure-8 cast-on (scroll down). I found this cast-on frustrating at first, but later I came to love it, and to see the advantage of knitting the mitten from the fingertips down, rather than beginning at the cuff. The advantage is that you can try on the mitten as you knit and make sure that it will fit. You can keep increasing at the tip until it's wide enough to fit the recipient's hand comfortably.
The other thing I like is that this cast-on allows you to have a seamless band that runs up the sides of the mitten, over the top, and down the other side, graphically separating the back of the mitten from the palm graphically. This band can be solid or striped. (You can see it on the green and red mitten at the top.) This is how stockings and socks are knitted in Eastern Europe: a similar band separates the sole from the instep and the front from the back of the sock. Anna Zilboorg has written extensively about socks from that part of the world, and presumably their design influenced her mitten designs.
AZ dyes her own yarn for her mittens, and I did the same. I had a big cone of sport weight yarn in white, and I dyed with my new stash of Greener Shades dyes, which are acid dyes with no heavy metals in them. I used a crock pot out on the porch, and it was very easy and satisfactory.