Feedback

You turn your amp up, and then stum an open string or strings and dont palm mute it. You have to have your amp turned up though. I don't know why though.

Hopefully I can explain another approach that I use. Which allows me to create feedback even at low volumes. I overdrive an all tube amp into a dummy-load or use an attenuator. This gives me that cranked rock guitar tone at any volume, utilizing a signal chain of equalizers and various other pieces of tube equipment I can generate controlled feedback and sustain at will. Without harming my hearing.

Another way is if you run a guitar amps preamp into distortion and the right amount of volume from the output section, you should be able to get control of feedback from certain area's within the sound field from the speakers.

Joe

If that still doesn't work; distort the sound, and turn the treble and the volume up high, but if it is too high, the feedback may became uncontrollable. Like forrok star said the certain areas in the sound field to get feedback they are the "hot areas "which is directly in front of the speaker, and close to amp around the sides. So you might have to move around a little.