Best AR15 Modifications for Accuracy?

What modifications have you found most effective for improving the accuracy of your AR15 or its variants?

All depends on what your AR is and your skill level 

I have a LaRue AR that is stupid accurate and the best thing I did was a case of high quality ammunition and shooting 100rds a day max getting to know the rifle 

but I was also getting tips from long range shooters at the same time