Reputation among Bandits spans from -100 to 100, you can raise it by usually submitting to them or in some cases dominating them, but using love potion is the best option, since it raises the reputation by 25.
Bandits' morale spans from 0 to 100 and this is what your interested to improve, you can raise it by going to the camp and take from Jester the job "A bitch for the thugs" or having the talk "give money to the bandits".