By bus:

Local and International Bus Schedule

Trebinje - Banja Luka: 10:00 (every day), 21:00 (Tuesday, Thursday, Sunday)
Trebinje - Mostar: 06:15, 10:00, 14:30
Trebinje - Sarajevo: 06:00, 07:30, 11:00
Trebinje - Lukavica: 05:00, 07:30; 11:00; one-way ticket costs 24,50 KM
Lukavica - Trebinje: 07:45, 13:00, 16:06
Trebinje - Dubrovnik: 10:00 (except on Sunday); one-way ticket costs 5 KM
Dubrovnik - Trebinje: 13:30