After trying multiple things to get good sensor readings (such as using resistors, capacitors, etc...) I came accross a Blog entry by majenko, who uses the internal reference voltage to get accurate readings.
double voltage = getVoltage(A0);
Serial.println(voltage, DEC);
0.7257656097
To print some additional debug information to the Serial, set the debug flag to true:
getVoltage(A0, true);
VCC: 5.161, raw reading:144.000, raw voltage:0.703, adjusted voltage:0.726