In De Rerum Natura, Lucretius espouses his interpretation of Epicurean philosophy, which is a materialist philosophy encouraging the individual to find happiness by understanding the phenomena of the physical world, rather than dwelling on superstitious or religious dogma.
Lucretius argues that all causes and effects in the world can be described by natural, physical laws. Therefore, it is illogical to suggest that a god, or gods, divinely interfere in our lives. It is therefore illogical and a waste of mental effort to worry about divine intervention. Lucretius also supposes that if a soul/mind can not exist independent of the body (when the body dies), then it simply ceases to be when the body dies. The individual will not be aware of anything; he/she will not be in any afterlife, he/she will just not exist anymore. And it is illogical to fear nonexistence because being nonexistent, one will not experience anything, fear or otherwise.
Therefore, in order to be happy, people should not worry about divine intervention or divine plans. They should also not worry about or fear death. Instead, people should focus on understanding the world; this would be the best strategy to find happiness. Lucretius and Epicurus argued that being free from worrying about gods and death would lead to a state of peace and freedom ("ataraxia").
For then, by such bright circumstance abashed,
Religion pales and flees thy mind; O then
The fears of death leave heart so free of care. (Book II)
Lucretius also describes the motions of atoms, saying that they swerve ("clinamen") at indeterminate times. This swerving leads to collisions, cohesion, and is the reason things exist/interact. Lucretius also states that this is evidence (analogous or literal) of free will.
Whence this free will for creatures o'er the lands,
Whence it is wrested from the fates, this will
Whereby we step right forward where desire
Leads each man on, whereby the same we swerve
In motions, not as at some fixed time,
Nor at some fixed line of space, but where
The mind itself has urged? (Book II)
So, the keys to happiness are to forget about divine intervention, stop fearing death and/or what might exist or not exist after death, to learn about how the world works (thus showing that there is no divine intervention), and to understand how the movement (swerving) of atoms shows how free will is a force living things can use to pursue their happiness. This is quite prescient in terms of describing the randomness of (sub)atomic movements, accidentally prescient or not. In the end, this recipe for happiness and a good life comes down to responsibility and self-reliance. The individual must focus on the physical world, the here and now. He/she must not waste time worrying about gods or death. We must appreciate things as they are, while they are here, while using free will and knowledge (science, the arts, etc.) to create happiness.