sounds like the synchro is going bad or a slightly bent shift fork....

 

I was having the same problem not too long ago. I took it to a transmission shop and had them look at it and asked them what to do. They said to put the car in 1st or 2nd gear before going to reverse because on my transmission the reverse isn't sync'd, so you first have to put it into a forward gear to stop the plate fromspinning and then it goes into reverse with no problem. If it goes into gear fine while driving it then there is nothing to worry about as far as damage they told me.