Make sure you're not gripping the shifter like a baseball. Use only a cupped palm, this will ensure the centering spring is working for you instead of you needing to manhandle the shifter.
I'll go into more detail on what I mean. When in first to shift to second you should only need to palm the shifter on the right side (thus pushing it left) and slide it back into second. From second to third the centering spring is strong enough to go right into third if you simple push straight up (don't push right, that will take you into 5th). Third to fourth is easiest, it's just straight back. Fourth to fifth is up with rightward pressure. Fifth to sixth is rightward pressure and back.
Note that the centering spring is strong but not super strong so if you shift to fast from say second to third (pushing straight up) you might hit the shifter gate wall and miss third. I fixed this by getting a new shifter. The centering spring in the steeda tri-ax shifter is really nice. You can throw it from second to third super fast without hitting the shifter gate wall.
'14 Grabber Blue Mustang GT
Manual, Premium, Steeda Tri-Ax shifter w/ red bracket, K&N CAI