Look through the components of your game, for the best bits. Which things are most fun, or strategic?
Add more of them. However, once you start adding these things everywhere, it's probably time to build them into the game at a higher level. Promote them from being objects, to being their own rule/concept/icon.
In Radlands, cards were initially all just in a row. It didn't matter where you put them. However, I made a Watchman card. He could be placed in front of another card, and the opponent would have to attack him, before they could attack the card behind him. This turned out to be such an interesting ability that I made more such cards that could protect a card behind them. I called this ability "protection". Eventually, I gave so many cards this protection ability, that I promoted protection to be a game rule — every person could protect.
In my gangster game, there are four different-coloured dice you can roll. I made a few locations that allow you to roll two specific dice, see what you roll, and then choose which one of those two rolls you want to enact. Choosing between two die faces on different dice is a very interesting decision, despite it being such a simple concept. I really liked these locations, but there were only six of them — one for each different pair of dice. I decided to promote this "roll two dice and choose one of the rolls" to an icon. It's now a die, split down the middle, with each half being a different colour. Because this is now just a simple icon, I can pair it with other effects, or have two of them on a location. This lets me use this cool effect on a huge number of the game's locations. (I then tried promoting it again, such that all die rolls are these "choice" rolls, but that was too much.)
In my gangster game, I created numerous cards that placed money or health tokens on various locations. Each of these cards had to specify that whoever went to that location got the tokens. Eventually, I just added this as part of the rules, and my cards got a whole lot simpler.
In my gangster game, there are criminal "profiles", that give you a special ability. There are many of these, and you play as one each game. However, there are special power cards that are available in every game. These are important, and I wanted some more of them. I took some of the coolest profiles, and promoted them to power cards. Now, those cool things are in every game.
Demotion is the opposite. However, sometimes you have a fun rule, but the game just doesn't need it. Rather than abolishing it, you can just demote it back into a game element, like a card.
In Radlands, you could pay one water resource to rearrange all your people into a new formation. It was an interesting rule, but players rarely used it. The rule was demoted back to being a single, interesting card, called High Ground.
Also, if an effect becomes too rare, it should be demoted.
Radlands had three damage icons — "damage anything", "damage a person", and "damage a camp". After some changes, the camp-damaging icon was present on only two or three cards. I removed it from those cards, and just replaced it with the regular damage icon plus the text "...a camp".