There are a few ways of increasing your Presence permanently. Some are tied to your Legacy, and thus affect every character you have or will make on a given server.
1. Datacrons. There are several datacrons that boost Presence. This method is not tied to your Legacy, so each character must find them. I use this site for my datacron hunting needs.
2. Level a Human to 50. Because Humans have the ability to be any class from the beginning, the reward for reaching level 50 as a Human is a 100 point Presence boost given to your Legacy.
3. Complete all quests and conversations for as many companions as possible. Each companion that you finish grants a 10 point Presence boost to your Legacy. With 40 companions across the 8 classes, this totals to 400 potential Presence points. You can view which companions you are done with on the Imperial/Republic Class tabs of your Legacy window. (Note 1: you'll need over 9000 affection for most if not all companions to finish with them, as well as progression in your Class story.) (Note 2: Bonuses given for completing a companion of a given Role (Healer, Melee Tank, Ranged DPS, ets.) do not stack. Thus finishing both Theran Cedrax and Doc will grant 20 Presence, but only one boost to Healing Received.)