I've no idea if Darren has done any urbex, so I'll offer my tips based on how I do it:
1) Do internals first, externals afterwards (in case you get busted - it's easier to revisit and do externals)
2) Explore different compositons - go wide angle, get close up, lie on the floor (if it's safe, and not covered in glass, needles, etc), look up at the ceilings, frame photographs looking through windows and doors.
3) Look for the evidence of past human activity - old signs, documents, etc.
4) Light and shadow - smashed in walls, doors and floors can let in light where it didn't used to - can anything be made of this?
5) Look how nature is taking over - plants growing through walls, etc.
6) Consider bracketing if inside as you may have areas of extreme shadow and light.
7) Reflections - sometimes places with no roof or a leaky roof can give some interesting internal reflections.
8) Is there a single shot that really captures the essence of the place?
9) Editting - consider choosing 5-10 shots that tell the story, that show different facets of the place.
I got commissioned last year to do an urbex style shoot on a mobile phone, have a look at the pictures here:
http://www.theviewfromthenorth.org/london-road-fire-station-samsung. I'm not posting these for critique, but as a recent example of how I personally photograph abandoned places.