Many shower curtains come with features to reduce the shower-curtain effect. They may have adhesive suction cups on the bottom edges of the curtain, which are then pushed onto the sides of the shower when in use. Others may have magnets at the bottom, though these are not effective on acrylic or fiberglass tubs.
It is possible to use a telescopic shower curtain rod to block the curtain on its lower part and to prevent it from sucking inside.
Hanging the curtain rod higher or lower, or especially further away from the shower head, can reduce the effect. A convex shower rod can also be used to hold the curtain against the inside wall of a tub.
A weight can be attached to a long string and the string attached to the curtain rod in the middle of the curtain (on the inside). Hanging the weight low against the curtain just above the rim of the shower pan or tub makes it an effective billowing deterrent without allowing the weight to hit the pan or tub and damage it.
There are a few alternative solutions that either attach to the shower curtain directly, attach to the shower rod or attach to the wall.