I´m no expert, but I had the same problem, so I do know what my physical therapist told me.

First off, tight muscles and strong muscles are two different things.  Apparently, sometimes muscles are tight BECAUSE they´re weak.

Anyway, I had glutes that wouldn´t fire on deadlifts, squats, nothing.  What I was told to do is activate them before every workout, activity, etc.  Doing bridges on your back, like this http://www.youtube.com/watch?v=8bbE64NuDTU worked great for me.  Now, the most important thing is to focus on doing them with the glutes and not letting the hamstrings and lower back take over.  Playing around with foot positions and pushing your knees out a bit should help.  Make sure your weight is on your heels.  You can also do these on one leg.

Then to get them strong I like to do barbell hip thrusts (google ´m).  Same thing here, play around with foot positions and make sure your weight is on your heels.

Now, a year later, my glutes know when it´s time to work, and they fire when needed.  And, most importantly, they´re a lot stronger, giving me better posture and helping me on the big compound lifts.

Hope this helps!