Prepare where your next resource is going to be. Plan ahead for when your gold mine runs out, and allow yourself a gradual farm transition, etc. Have army or defences to protect the new location.
Don't let yourself get population blocked. Build houses ahead of time, possibly by control grouping a villager and ensuring they are always building.
Scout. Find relics you can use. Find ones you might want to deny from your opponent. Find where they might be moving for their second gold mine etc. so you can send army there. Find out what unit comp they are going so you can counter it. If they're going 2TC, that's an opportunity to attack their builders.
Wall up your weak spots or place defensive buildings to protect it. "Cheeky" small walls that don't fully block it off but force your opponent to walk around a long wood line are a good cost-effective option.
Raid your opponent. Counter-raiding while they raid you, or sending a small number of myth units to the enemy's gold mine while most of your force attacks somewhere else can be especially effective, as they are less likely to notice it as their attention is elsewhere.
Spend your res! 5โ6 construction buildings in the mid game, and at least 10 in the late game are a good rule of thumb. If you are floating a lot of res, build more production, or research more techs.