I always knit both my sleeves at the same time - two skeins, two identical pieces, on two single-pointed needles. This way, even if I make a mistake with increases, they are still identical. Also works for two fronts, you just need to remember to reverse the shaping.
Lately I have been knitting more and more on circular needles - easier on the wrists, takes less space. And - you see the problem, right? With circular needles the work shifts around when you put it down, and it is hard to know where to start when you pick it up again.
Eureka! Attach the two identical pieces with safety pin/stitch marker/whatever, so that the two pieces resemble one, with a right and a left side, and a right edge and left edge. You will only lose your place if you stop in between the pieces, so don't do it!