In these days, Lethargy-&-Laziness may commonly be induced by self-destructive behaviour such as:
Alcohol, drugs, dope, pills, marihuana, sniffing, over-eating, excessive masturbation and night-living.
Such often life-long chemical or behavioural causes of Lethargy-&-Laziness should be eradicated...

As long as this is not achieved one remains a phlegmatic & apathetic zombie, drifting downwards!!!

First priority: Noticing Lethargy-&-Laziness (thīna-middha) emerge can make it evaporate:

Herein, Bhikkhus, when Lethargy-&-Laziness is present in him, the bhikkhu notes & understands:
There is Lethargy-&-Laziness in me, and when Lethargy-&-Laziness is absent, he notes & understands:
There is no Lethargy-&-Laziness in me. He also understands how unarisen Lethargy-&-Laziness arises.
He understands how to leave behind any arisen Lethargy-&-Laziness, and he understands how left
Lethargy-&-Laziness will not arise again in the future.                                               MN 10

 

What is the feeding cause that makes Lethargy-&-Laziness arise?
There are boredom, apathy, tiredness, lazy stretching of the body, heavy drowsiness after meals,
mental sluggishness, frequently giving irrational & unwise attention to these mental states, this is the
feeding cause of the arising of unarisen Lethargy-&-Laziness, and the feeding cause of consolidation
and deepening of Lethargy-&-Laziness, that already has emerged.              

                                                                                                                                           SN 46:51

What is the starving cause that makes Lethargy-&-Laziness cease?
There are the events of Initiative, of Launching into Effort, and of tenaciously Enduring Persistence,
frequently giving rational & wise attention to these three mental elements, is the starving cause for the
non-arising of unarisen Lethargy-&-Laziness, and the starving cause for the arousing and stirring of
Lethargy-&-Laziness, that has already emerged.    

                                                                                                                                            SN 46:51
The Supreme Ideal:

Before the Buddha sat down to meditate in order to attain enlightenment, he made this determination:

May just all flesh and blood of this body dry up into a stiff frame of only bones, tendons and skin...
Not before having achieved, what can be achieved by manly strength, manly power, manly energy, 
will I rise from this seat...                                                                             MN 70
 

Some advantageous reflections to return to whenever Laziness threatens:

 

How to stimulate the mind:
How does one stimulate the mind at a time when it needs stimulation?
If due to slowness of understanding or due to not having yet reached the happiness of tranquillity,

one's mind is dull, then one should rouse it through reflecting on the eight objects stirring urgency.
These eight are: birth, decay, disease & death; the suffering in the hell, demon, ghost & animal world;
the suffering in the past rooted in the round of existence; the suffering of the future rooted in the
round of existence; the suffering of the present rooted in the pursuit & search for food and living.
                                                                                                                                           Vism. IV,63
Perceiving the suffering in impermanence:
In a bhikkhu, who is used to see the suffering in impermanence and who frequently reflects on this,
there will be established in him such an acute sense of the danger in laziness, apathy, inactivity and
lethargy, as if he were threatened by a mad murderer with drawn sword.
                                                                                                                                            AN 7:46

Lethargy-&-Laziness is an inner mental Prison:

Just as when a man has been forced into prison is Lethargy-&-Laziness, but later when he gets
released from this (inner) prison, then he is safe, fearing no loss of property. And at that good
he rejoices, and is glad at heart... Such is the breaking out of Lethargy-&-Laziness...

Another person has been kept in jail during a festival day, and so could see none of the shows.
When people say: Oh, how fun was this festival! He will remain shy, mute and silent. Why?
Because he did not enjoy any festival himself... Similarly is prison of Lethargy-&-Laziness...
Another person that once had been in jail on a festival day. But when freed and celebrating the
festival on a later occasion, he looks back: Before due to my careless laziness, I was in prison on
that day & could not enjoy this festival. Now I shall therefore be Alert & Careful. Since he remains
thus Alert & Careful no detrimental state can overcome his mind. Having enjoyed the festival,
he exclaims: Oh, what a fun festival it was! Such is the freedom from Lethargy-&-Laziness...
 

The remembrance of Death: To-day the effort should be made, Who knows if Death comes tomorrow? MN 131	Easy is the shameless life now. Easy is it to be bold, retaliating, lazy, uninformed & wrong-viewed. Dhammapada 244
Rouse yourself! Sit up! Resolutely train yourself to attain peace. Do not let the king of death, seeing you lazy, lead you astray and dominate you. Sutta Nipāta II, 10	Carefulness is the way to the Deathless. Carelessness is the way to death. The Careful die not. The Careless are as if already dead. Dhammapada 21

How to get to the opposite good mental state: Enthusiastic Energy (Viriya) see here:

Enthusiastic is Energy: http://What-Buddha-Said.net/drops/Enthusiastic_is_Energy.htm

Arousal Get Up and Going: http://What-Buddha-Said.net/drops/Arousal_Get_Up_and_Going.htm

More on this common self-destructive behaviour: Drinking & Drugging:
http://What-Buddha-Said.net/drops/II/Curing_Lethargy_and_Laziness.htm
http://What-Buddha-Said.net/drops/III/Optimal_Observance_V.htm
http://What-Buddha-Said.net/drops/IV/Blurred_and_Blinded.htm

See also:
http://What-Buddha-Said.net/drops/IV/Suitable_Substitution.htm