I would basically agree with what has been said, plus:
2. For the dot on the E6B either use chinagraph pencil and eraser, or non-permanent Overhead Projector Pen plus paper tissue.
3. DR means flying a heading for a time and then spotting something on the ground which allows you to make a 'mid-course correction'. Except there may not be anything at the midpoint or you may risk busting controlled airspace before you get there. So instead of blindly correcting at the midpoint, plan to make the correction over a suitable landmark. Landmarks with a well defined width are best for estimating angular errors. And if you are good, the nose will be in the way, so start looking 3-5 minutes before you get there.
4. Definitely take a GPS, and use it as much as you are allowed to. Also use VOR/DME if allowed. Navigation is easy when you know roughly where you are, most of the time. Aim to have visual nav, VOR/DME and GPS all telling you the same thing. If you get lost, try to tell somebody over the radio, you may get radar/VDF.
5 GPS and autopilot, (and a pilot flying a VOR track for that matter), are every bit as efficient as a DR pilot flying a perfect wind-corrected heading. In every case the heading is adjusted empirically until the track is good. In fact unless the wind forecast is perfect, the DR pilot is the least efficient, because he doesn't even discover his track until he makes his correction, whereas the autopilot and VOR tracker start correcting immediately.