build up
With or without an object
Increase gradually, or make something increase
She built up her strength slowly after the operation.
With or without an object
It can be used with no object, and it can also take one. With an object it is usually separable: turn the volume up, turn it up.
The particle: up
This verb belongs to the group ‘getting bigger, louder or faster’.
Practise it
Add it to your daily practice or see every verb with up.
Further reading: build up on Wiktionary (CC BY-SA).