Formal:
* Regularly
* Steadily
* Uniformly
* Invariably
* Persistently
* Reliably
* Continuously
* Unwaveringly
* Enduringly
Informal:
* Always
* All the time
* Every time
* Without fail
* Like clockwork
* On a regular basis
More specific:
* Repeatedly: emphasizes the action happening many times
* Habitually: implies something is done out of habit
* Persistently: implies a strong determination to continue
* Unfailingly: emphasizes the lack of exceptions
The best synonym will depend on the specific meaning you want to convey.