Speaking from personal experience, stretching and extenders seem to be the most beneficial in terms of PE.
There have even been some studies that suggest extenders MIGHT work, whereas other forms of PE haven't really been tested thoroughly. Extenders have the unique advantage that you can wear them all day if you aren't active (I have a noose one, and it's really uncomfortable, and I can barely wear it an hour a day, and have to take breaks from it often, vacuum extenders are ideal but very expensive).
But I have had successes with manual stretching. But just like everything else, it takes a lot of time and dedication. consistency trumps intensity everytime in this case.