After suffering a hip/low back injury, I was identified with a glute activation issue (by my physio who is a biomechanics expert and high level athletics/AFL strength coach). I never felt squat or deadlift workouts in my glutes, now I do and I believe the problem is completely resolved.
I did no heavy assistance work, I just used activation exercises. Once activated, I reset all my squats and deadlifts and work on ensuring I was using hip drive all the time (ie staying on my heels in squats, thinking about driving my hips forward from the floor with deadlifts), and the glutes caught up by themselves.
The activation drills I was given were:
Glute bridges (bodyweight), 3 sets of 10
Prone single leg heel lifts (with knee at 90' degrees), 3 sets of 10 each side
Every day for 6 months.
Once I got stronger, I started doing single leg glute bridges.
The important thing is not worrying about how difficult the exercise is - they are simple - the focus is on doing these exercises properly so you don't use your low back, quads or hamstrings to do the work. You must use your glutes. Clenching your butt and pausing for 3 seconds at the top of the activation exercises can help.
Here's a really good article on the topic:
TMUSCLE.com | Dispelling the Glute Myth
I didn't do the hypertrophy phases etc. In 3-6 months got a very noticeable hypertrophy and strength increase in my glutes just from combining the activation drills with powerlifting.