Well to keep straigth u have to put ur dominate hand on bottom and ur nondominate hand on the side and shoot. hope i can help some if u have any questions just ask me.

In my opinion you've got to keep your back straight and have a good elbow ngle. plus this wont work at all if you cant shoot like this. if it bothers you, you cant shoot straight. if it doesn't work it never will, as simple as that. everybody has a free frow stance and you can change it, just not by force. your free throw stance will grow on you, and you have to learn how to make the best of it.

When shooting free throws there are a couple of steps you have to follow. First of all i am not sure what your coach means by staying straight? When I say something to my team about shooting it generally is that they need to keep their elbow in to help striaghten their shot.

First. step up to the line and set your feet. Your stance should be as wide as your shoulders, no more,, no less.
Second. Get used to doing the same pre-shot every time. If you bounce the ball 3 times and then spin it in your hand before you shoot for example, do that everytime to create a routine.
Third. make sure when you get in your shooting form that your elbow on your shooting hand points straigt down at your toes. And that your shooting hand and arm is completly straight up and down.
Fourth. Get the ball on your finger tips and off of your palm (this creates more power. Bend your knees and as you shoot make it all one motion. Make sure you follow through with your hand towards the goal. Swish!!!!