I was looking through the latest AAA "Westways" magazine and came across a little ad for "OneDerWear."
OneDerWear is an ultra-light disposable underwear created for traveling. Designed to provide the utmost comfort and convenience, OneDerWear disposable underwear is 100% cotton and ideally packaged for maximum space efficiency. Each package contains five compact pairs of individually wrapped disposable underwear that can fit in the palm of your hand. With OneDerWear, you simply wear and toss! By the end of your trip, you'll be surprised to find plenty of luggage space for gifts and souvenirs.
They also sell disposable socks and "Planesheets," which are full seat covers for use on an airplane.
I have found it helpful to do the stretching exercises my physical therapist recommends. Do some core strengthening exercises, such as the ones highlighted at The Mayo Clinic. By keeping your core strong, your back won't put itself in a position to cause that sciatica pain you are having.