The two 'if' statements with && should be inside the first 'if' statement so that they are only tested when a serial character is available.
void loop()
{
buttonState = digitalRead(buttonPin);
if (Serial.available() > 0) {
incomingByte = Serial.read();
if (incomingByte == 'H' && buttonState == HIGH) {
myservo.write(180);
delay(1500);
digitalWrite(ledPin, HIGH);
}
if (incomingByte == 'L' && buttonState == LOW) {
myservo.write(90);
delay(1500);
digitalWrite(ledPin, LOW);
}
}
}
Pete