Try to make it fun.
Don't get yourself too worked up or stressed, she can pick up on that
Depending on her personality - dare her "I bet you can't ...", or playing off of funhusker - measure her urine and challenger her to make more next time
Boys are interesting with the aiming thing going on. My niece tried to train her son. Always missed to the left. Turns out his urethra "took a left turn" - it did not point straight out!