Couple tricks I used to use (pre-MGW that is) is what's already been mentioned..

The 'release the clutch, then re-engage the clutch' method and the 4th to R jump.

 

Hope this helps you.

Here is a previous post I found I remember reading before:
https://mustangforums.com/m_2429020/tm.htm

StartMeUp- +1 to howarmat. There is a safety interlock in the tranny (at least the GT's manual) that prevents accidentally engaging reverse. I remember reading about it my the owner's manual. If memory serves, you can go from 1st or 2nd to reverse but not 3rd, 4th or 5th to reverse.

 

Thanks for all the help.

 

That is normal from what I have been told and it happens to many people. Try placing in fifth first and then move down to Reverse. Works for me when acting up. Sometimes just move up a little in first than go to neutral and then proceed to reverse and that helps as well!

 

+1 on moving a little forward in first then reverse. This happens to me every once and a while. All I do is move forward just a bit and then the syncro gods are in line and off I go. Give that a try it will help.