Your pet should be able to turn around, sit and/or lie comfortably on a towel or other soft, absorbent material inside the carrier.
Bring favorite toys, and only feed your pet a small meal before boarding to reduce the likelihood of air sickness.
The American Veterinary Medical Association cautions against the use of sedatives in most cases, as these drugs may alter your pet's natural ability to balance and maintain equilibrium, can have unanticipated side effects and may exacerbate existing and/or undiagnosed medical disorders.
Your pet may benefit from an anti-nausea drug if he or she is predisposed to motion sickness. Ask your vet for a prescription.