Getting good feedback

Step one:

You have to be LOUD to get feedback (unless you use an effect to get it at low volume).

Dammit, paults! You always beat me to these threads!

Like he said, volume is important for natural feedback. And for controlling it, use the volume knob on your guitar.

It's much easier if you have a hollow body guitar and can get close to the speaker. Putting a speaker cab up on a stand I've been amazed at how low the volume can be and still get feedback. Solid body is a very different story.

Feedback is a physical phenomenon.
You want your guitar to vibrate in (mathematical) harmony with the note you want to feedback -
so as said, you need a lot of volume and preferably a resonant guitar to make it vibrate, and the correct distance between you and the speaker to have the correct note (harmony).
For example, getting feedback on a low A would be easiest if you were standing 3 meters away from the speaker.
Higher notes call for smaller distances.

A good studio trick is to lean your headstock against a studio monitor. It's not 100%, but it's a good, simple workaround.

Quote from Quitty

For example, getting feedback on a low A would be easiest if you were standing 3 meters away from the speaker.

Actually, I think it'd rather be at about 0,8, 2,2, 3,9 etc meters, as the air movement is greatest at 1/4 into the wavelenght.

I use a Sustainer pickup in a Jackson PC-1. Feedback on demand regardless of volume and stops when you damp the strings with no screeching. Can even get feedback on clean or low gain profiles.

Turn everything up to 11, yes 11 that's right because it's one louder lol

I've gotten low-volume feedback by holding my headphones up near my pickups.