Kneel down to your amp and go nuts, basically.

Good way to get a really dirty sound out of it is with some delay and a lot of whammy-work
Turn the volume up, and move your guitar nearer to the amp; it's the magnets in the pickups that cause it.

I can have no feedback while sitting no less than half a metre away, but that's because the volume's on 1. Turn it up to 6, and I need to stand back two metres, lest my ears be destroyed.
Other option is a boss DF-2 pedal, they dont make these anymore but you can pick them up on ebay for about £50. They work like a simple distortion pedal, but when you hold the pedal down it goes into singing feedback, with a control to vary the amount of overtones. Or alternatively play with your amp on high volume - single coil pickups help as well.
turn your gain up all the way
uh...bring volume to desired level... the higher the farther youcan move away from your amp
so yeah uh...
dont play anything..and it'llfeedback
also, fret 10 on the low E, 11 on the A, and 12 on the D and that should begin to uh..feedback
idk thats only for me though