I've seen anti sea-sickness bracelets, but the mechanism of action is unclear to me, as well as the physiological reason behind them.
According to the CDC, there's been no studies definitively proving they work.
There has been a study that resulted in no definitive proof that they do work.
A description of the supposed mechanism of relief: (From the above-linked study)
Acupuncture is the practice of stimulating points on various meridians in the body with needles and is generally performed by a trained acupuncturist. The Neiguan/P6 point is the sixth point on the pericardial meridian and is believed to be the point where nausea and vomiting are controlled. The P6 point is located between the flexor carpi radialis and the palmaris longus tendons, one sixth of the distance between the distal transverse crease of the wrist (top crease, closest to the hand) and the cubital crease (elbow crease). According to the ancient Chinese meridian theory of acupuncture, life is balanced if the positive and negative energies are equal. These energies flow through invisible channels (meridians) in the body, connecting the points of the body and visceral organs. By stimulating these points with needles, the acupuncturist attempts to restore the balance of energy flow throughout the body. For nausea, negative energy in the body is first redirected away from the heart by stimulating the right P6 point and then more positive energy comes into the body through stimulating the left P6 point, the one closest to the heart. Purportedly, the balance of the life forces, yin and yang, are restored and nausea is controlled.
They also have been tested by Mythbusters in 2005 and found not to work (unsurprisingly).
Wrist straps: They wore little gray wristbands that are 'Barry Manilow's choice.' Adam was sick within 90 seconds. Grant got sick as well. They've gotten pretty quick with bringing a bucket to Grant.
It's not exactly definitive proof, but this meta-study of seven trials seems mildly positive:
Four of seven acupressure band trials supported the positive effects of acupressure, whereas three acupressure band trials yielded negative results regarding the possible effects of acupressure; however, all the studies with negative results had methodological issues. In contrast, one quasi-experimental and two randomized finger acupressure trials all supported the positive effects of acupressure on CINV control.
"Review of acupressure studies for chemotherapy-induced nausea and vomiting control", Lee J, et al. J Pain Symptom Manage. 2008 Nov;36(5):529-44.
That's 7 of 10 in favor, and 3 studies "with methological issues" against. Technically these are for chemotherapy, not motion sickness, but I'd presume nausea is nausea.