"1. change the program, the input is valid is only it is the same for at least 100ms"
This will keep you from picking up short noise pulses on the line.

"2. connect a 100 ohm resistor fronn +5 to the input pin"
Reduce the impedance of your control line to reduce the noise voltage generated by noise engergy coupled to your control line.
"3. connect a capacitor 1uf from the input pin to the ground"
Noise suppression again
"4. put the switch between the input pin and ground"
Standard switch connection for the '51
"5. connect a tantalum capacitor (100uf)
from the +5 which is located near the 100 ohm resistor to the ground"
Absorb the noise enregy discharged through the 100 ohms.

Hope this helps

Erik