As Mark has suggested, stirring may be the best approach for a paint that has been standing for a long time. Another benefit if using just the stirring method is the residue on the inside of the screw top doesn't spread to the meeting surface between the top and the edge of the bottle. Over time a buildup of paint forms between the top and the bottle and makes it hard to remove the top. Since I have stopped shacking small paint bottles to mix the paint and stirring them only this problem has all but stopped.