My technique on the hard/extreme tracks suuuuuucks. How am I supposed to clear that barrel up the hill in Down in the Dumps? Or jump through that pane of glass on the track after it?
For the pipe on the hill in Down in the Dumps, you need to get good traction on the rear wheel while leaning forward (feather the throttle, don't max it), then let the left stick go to neutral (you're still in the forward lean posture) before hitting the pipe so you go into a wheelie. the rear wheel should hit the pipe first. when the rear wheel hits, push forward again so you get on top of the pipe, then let the stick go to neutral again so your rear wheel touches down so you can get off the pipe.
For the glass window in the warehouse, you need to jump from the platform below the glass, it'll be pretty much impossible to go directly from the high platform. if you're not going for platinum, you can drop down to the lower platform first, back up to the beginning of the angled platform while leaning forward, then floor it and you should go through.