"Be present" is the most repeated and least useful piece of advice in the modern wellness canon. It is repeated so often that it has lost all meaning. So here is something more specific.
The research on what actually increases present-moment awareness points to a few consistent factors: novelty, social connection, and physical engagement. Routines are the enemy of presence. Not because routines are bad, but because the brain stops paying attention to things it has seen before. Introducing novelty, new places, new activities, new conversations, forces the brain back into active engagement with what is actually happening.
Social connection is the other major factor. The periods of life that people remember most vividly and most warmly are almost always periods of significant social connection. The apartment with the roommates, the years when the family was all together, the job with the team that actually liked each other. Investing in relationships is not just a source of happiness in the moment. It is an investment in the quality of your future memories.
And finally: write things down. Not as a diary, necessarily, but as a record. The act of writing about an experience, even briefly, even badly, forces you to pay attention to it in a way that passive experience does not. It also creates the raw material for future memory, which means that the good old days you are living right now are more likely to be remembered as such if you have some record that they happened.
You will not know the good old days are happening. That is almost certainly true. But you can make it slightly less true, with attention and intention, than it would otherwise be.