With my son we spent some time with no pants or underwear, to increase body awareness. I took him out to the back yard with no pants or underwear on him and we played out there for long enough that he peed a few times (we did this for several days, I don't remember how many exactly).
Pretty soon it got to be fun for him to "water the plants," and he was pretty clear about when pee was coming out. It also gave me a chance to figure out approximately how long he could go between pees, so when we went back into the house I could set a timer and have him go to the potty when it was time (originally once every 45 mins to an hour).
My son didn't start going by himself until he'd been "potty trained" for about four months, AND he felt like he could do each step by himself (e.g. Pulling down his own pants was a challenge at first). I'd say the only downside to the backyard method was that he kept asking to pee in the backyard for a while after we stopped doing that :) but I kept reminding him that the goal was to pee in the potty like mama and dada, and he eventually got over it.