What I did is to have a global variable called difficultyFactor, I incremented it every amount of time, then I used that factor to increment speed or anything I need to increment.
ex
Lets say:
difficultyFactor = 0
regularSpeed = 100
Then in my game sheet:
Speed = regularSpeed + (regularSpeed * difficultyFactor/10)
This wont modify the speed because difficultyFactor is 0 at this point, then after some time:
difficultyFactor = 1
Speed = regularSpeed + (regularSpeed * difficultyFactor/10)
This will increment the speed by a decimal fraction giving us 110 for speed
You can play with those values.
Also I needed to spawn some enemies at a certain difficulty, so I just did:
if (difficultyFactor > 5) spawn Enemy
I hope it helps