The proper use of a leg immobilizer is very important. You may be wearing a splint or brace to immobilize an unstable joint or a fracture to minimize stress on it and relieve pain while your injury or surgical site heals. It is vital to make sure it stays in place or it cannot do its job, subjecting your leg to further damage. Knowing your immobilizer is correctly fastened and fitted will make you less nervous about moving around.
Place the brace slightly higher than your knee. This will allow for some wiggle room as far as slipping goes.
Pull the Velcro straps of the leg immobilizer tight but comfortable. You don't want to cut off your circulation.
Find a local orthotics supply store. A specialist there will be able to properly fit the brace to your leg. If you have been issued a generic brace that doesn't fit you well, it may require a little bending or cutting to customize it so it doesn't fall down.
Tips
Your brace will probably slip a bit just due to gravity, so you may need to adjust it from time to time.