Mistakes

The typical mistake made by a non-practitioner is to believe that experience can be permanently improved (i.e., that I can become happier) by rearranging bits of experience (i.e., doing stuff).

The mistake commonly made by practitioners is more subtle: it's that we can somehow improve experience (get more enlightened) by doing something (possibly very subtle) with our minds. There can be this underlying sense of improving our minds in some way. To make them calmer, clearer, sharper, etc.

But how would you do such a thing? Where is this mind you seek to improve, anyway? What properties does it have that could be improved upon?

This is why Mahamudra instructions spend a lot of time making you search for the mind and its characteristics.

Dzogchen instructor James Low comments (on a Dzogchen teaching from a master):

The purpose of this teaching is to give you confidence to trust that the mind is pure from the beginning. You cannot purify that which is already pure so don't waste time in that direction. ... The more you see there is nothing for you to do the more you find yourself relaxing.

And, after all (Thrangu R):

A relaxed mind is all that is necessary. Perfect meditation will arise in a perfectly relaxed mind. 

Tulku Urgyen R:

The antidote for exhaustion is, from the very beginning, to relax deeply from within, to totally let be. The best relaxation brings the best meditation.

We need the best relaxation. The difficulty comes from not having this. What becomes tired is the dualistic mind. 

And finally:

Our body is the Bodhi tree,
And our mind a mirror bright,
Carefully we wipe them hour by hour,
And let no dust alight.

That is the poem that was refuted by Hui Neng, who went on to become the 6th Zen Patriarch, with this gem:

There is no Bodhi tree,
Nor stand of a mirror bright.
Since all is void,
Where can dust alight?