I'm sure it's been posted and discussed but I can't find it. Anyway, in Warehouse you can ride the boxes up to the BFG. Once you get to the top level and before the arch that kills you, jump across to the ladder from your box. Climb up ladder with forward key.
Once you get the BFG you can jump on the belt after the arch and walk down to the battle suit power up in a little niche just before the laser wall. Just leave enough room to get the power up and get out again before you get boxed in.