How Do I Properly Ground my Arduino Leonardo using Capacitive Sensor?
The documentation for that library is recommending that you place a 100 pF capacitor between the sense pin and ground.

However, I personally suspect you will get the best results when the user's presence affects at small mutual capacitance between a sense surface and a ground surface.

One idea could be to have a large ground plane, and on it a smaller insulated "sense" plate.

Another could be to run two parallel traces in a spiral filling a large area

Do take care to secure you wires - they are part of your sensor too, and if they move relative to each other they will alter the baseline.  Any other mobile cables in the vicinity can have the same effect, especially if they contain a ground that can exhibit capacitance to the sense node.