How do the Pro's do it?

There is. Spend 59c at HD or Lowes for the plastic lips that snap on the rim. The ones that bend 1/2 way around the rim. They keep the lip clean. Buy a few! Just clean off before the paint hardens on them. Few other tips...some have already been mentioned...don't use a screwdriver to open the can or a regular hammer to seal it. They deform the lid. Use one of those paint can keys and a rubber mallet. Keep the rim clean and store your paint upside down. When ready to re-use, give it a good shake for 15 seconds. My paint lasts for years like this...no skinning or black gunk.

--Jeff