Logging to multiple destinations (serial & SD card)
The simplest thing you can do is to create a simple function that performs both tasks for you.  However, there are certain caveats that you should look out for.  The most basic function would be:

void log(const char *msg) {
    Serial.println(msg);
    myFile.println(msg);
}


However that is not the best way of doing it since that requires all your messages to be in RAM.  So you should really use the __FlashStringHelper type as well as an overload function so you can use the F() macro to save RAM:

void log(const char *msg) {
    Serial.println(msg);
    myFile.println(msg);
}

void log(const __FlashStringHelper *msg) {
    Serial.println(msg);
    myFile.println(msg);
}


And that will pick the right version of log depending on what you pass.  But what if you want to pass some variables, like numbers?  Overloading can help again here.  Another pair of functions now that take two parameters instead of one:

void log(const char *msg) {
    Serial.println(msg);
    myFile.println(msg);
}

void log(const __FlashStringHelper *msg) {
    Serial.println(msg);
    myFile.println(msg);
}

void log(const char *msg, int v) {
    Serial.print(msg);
    Serial.print(" ");
    Serial.println(v);
    myFile.print(msg);
    myFile.print(" ");
    myFile.println(v);
}

void log(const __FlashStringHelper *msg, int v) {
    Serial.print(msg);
    Serial.print(" ");
    Serial.println(v);
    myFile.print(msg);
    myFile.print(" ");
    myFile.println(v);
}


So now you have 4 functions called log, and they can be used in these different ways:

log("This is a message"); // Could be a char * variable
log(F("This is a message"));
log("The temperature is:", temp);
log(F("The temperature is:", temp);


And of course you can keep expanding and expanding the parameters in different ways for different combinations of parameters.  Just be warned that for some combinations of parameters the compiler won't be able to work out which is the right function to call (for instance if you have one that takes a long and one that takes a byte in the same place and you just pass a numeric literal), so you have to think carefully about how you arrange your parameters.