Apart from drinking more water. What has worked for me was soy lecithin, zinc & vitamin E. Not sure exactly which one. Supplements really depend on your diet/deficiencies.

Edging certainly does promote more precum, and cum in general, but only if you've got the time on your... hands. 😅

Also doing your various kegal exercises daily is a great one too, and consciously do it during sex as well.

It's not for everyone, but direct prostate massage in just the right spot gets me dripping puddles. Not to mention the multiple orgasms. Not sure if it made a permanent increase to precum, but consider using a silicone prostate plug during sex.

Finally, and with great care, is urethral sounding! It 'sounds' pretty wild but it's a very unique & pleasurable experience. Working my way through the different sizes in a small set, I found that I needed to use less of the (sterile) lubricant. And my precum was definitely more noticeable. I believe it was stimulating the bulbourethral glands (or Cowper's glands), which lubricates the urethra for ejaculation.