Rings can fit differently from day to day. Weight loss can affect how a ring fits, and fluid retention can cause an ill fit in the short-term. Seasonal temperature changes also cause fingers to swell and contract, changing a ring's fit. Wearing a ring that is too loose increases the risk of loss, and a ring that turns doesn't do justice to a beautiful stone. Having a ring resized is a permanent solution, but there are temporary fixes to make a ring fit smaller.
Purchase a ring guard to make a ring fit smaller. These small, tubular inserts are clear and won't significantly alter the appearance of the ring. They fit over the back of the band and fill excess space between your finger and the ring.
Have a jeweler install a metal sizer on the inside of the ring. These small I-shaped inserts are more discreet than the plastic ring guards and may feel more secure.
Make a disposable ring guard with clear packaging tape. Cut a 1/2-inch piece and roll it into a tight tube with the sticky side exposed. Fit the tube into the widest part of the ring band to fill the excess space. The tube will stick to both the ring and your finger for a secure fit (see Reference 1).
Place a small, clear rubber band on your finger under the ring to cause friction, keeping the ring from sliding off. Orthodontic rubber bands work for very slim fingers, while clear hair elastics fit better over larger fingers.