Commercial jets are typically pressurized to the equivalent of 6,000 to 8,000 feet of elevation. But according to a study, passengers begin feeling uncomfortable at about the 7,000 feet mark, right in the middle of that range:
we did find evidence that the level of hypoxemia manifested at 7000 to 8000 ft played an important role in the development of discomfort [...] we conclude that maintaining a cabin altitude of 6000 ft or lower (equivalent to a barometric pressure of 609 mm Hg or higher) on long-duration commercial flights will reduce the occurrence of discomfort among passengers.
According to the Wikipedia article on Cabin Pressurization, the typical cabin altitude equivalent for new aircraft is "falling", which "significantly improves comfort levels", so perhaps making sure to fly on newer aircraft is part of the solution:
The designed operating cabin altitude for new aircraft is falling and this is expected to reduce any remaining physiological problems [...] The 787's internal cabin pressure is the equivalent of 6,000 feet (1,800 m) altitude resulting in a higher pressure than for the 8,000 feet (2,400 m) altitude of older conventional aircraft [...] such a level significantly improves comfort levels.
So, is there a reliable way to check that my upcoming commercial flights will be pressurized closer to 6,000 feet than 8,000 feet? And is there any way for me to avoid booking tickets on high-altitude-equivalent flights so that I can be sure to avoid airplane sickness during my travels?
 Effect of Aircraft-Cabin Altitude on Passenger Discomfort, DOI 10.1056/NEJMoa062770