Set the stage for success. Have all your products like soap and shampoo lined up and the shower running. Sometimes you can just move them into the shower almost spontaneously.
Also check with a local home care provider or medical supply store. There are new products you can use to give a good sponge bath or soapless shampoo.
Get a note from the doctor saying that he or she is prescribing a shower or bath two times a week. An authority figure besides yourself may help.
Change the language. Call it a spa treatment or makeover.
Use their story life for inspiration. Get the person in a better mood by discussing their favorite subject before you bring up the shower (their home town, their favorite sports team, a loved family member). Sometimes you have to dig deep, perhaps mention that we need to get cleaned up and presentable in case the minister drops by or to go out for church.
Your family member may cooperate more with a paid caregiver who has special training and the confidence to get the job done.