For the parallel viewing type of drawings:
look straight ahead
hold the page at reading distance or arms length, but just below your line of sight
focus on a point far far away (like the next street). That will mean you are focussing beyond the room, and it takes a little practice.
Once you can hold this distant focus, move your arms up to bring the page into your line of sight. That should give you the 3D effect.
Holding your focus while moving the page up is tricky, until you have the technique, but once you see it, you will be able to do it again easily, anytime.
Alternatively, hold the page in sight, at arms length; then focus well beyond the page. Like the first technique, it takes a little practice, but it does work for 95% of people, and its easy once you get the technique to work.
Marcus