Get started. Then get out of your own way.

The single most important step in long-term savings is pretty simple. Start saving.

There are dozens of competing priorities for your financial resources, but precious few of them are more important as saving for the long-term. So start.

Once you've begun saving for the long-term, it's crucial keep that saving on track.  That task seems like it would be pretty simple, especially if you use techniques like automated paycheck deductions. But our brains are wired to sometimes overthink, and the volatility present in many appropriate long-term investments are tailor-made to trigger this behavior.

Don't just do something; sit there

Three factors cause long-term investors to make rash decisions or lose thousands of dollars along the way. 

Emotional Decisions - Disciplined long-term investing calls for deciding on an approach and sticking with it, regardless of the articles in the Wall Street Journal or the lead story on CNBC. Investors naturally begin to feel a bit of greed as they see fund prices (and their fund balances) creep up.  This greed - coupled with the 'wealth effect' where rising account balances drive increased consumption - can cause investors a euphoric feeling of self-assuredness which can lead to more buying. This buying happens at a higher price point (buying high).  As the market reverts to the mean, sinking prices replace the greed with fear, and investors can feel compelled to sell following a drop (selling low) to avoid further losses. Neither act serves the long-term investor well. The graph below tells the story:

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Risk Aversion. Many investors are uncomfortable with the large amplitudes swings inherent in a volatile investment, and thus shun this risk (and associated return) for less-swingy, lower returning investments. Since volatility settles out to be of quite little consequence over a long-term time horizon, avoiding the more-volatile investments simply leaves a lot of potential return on the table, shaving thousands and possibly hundreds of thousands off your wealth at retirement. Paradoxically, avoiding risk in long-term investing typically leads to a smaller pool of wealth, feeling far "less safe" in retirement than if you had assumed more risk along the way.  

More here: https://investor.gov/introduction-investing/basics/save-invest/gauge-your-risk-tolerance

Recency Bias.  Playing directly into the emotional decisions discussion, recency bias describes that events that happen more recently are easier to recall.  Once recalled, the 'availability heuristic' is a cognitive bias that overestimates the likelihood of things happening that are easier to recall.  Combined, this means we tend to easily recall recent events and assume they are likely of continuing to happen. This drives the greed emotion (this bull market is gonna last forever!) and the fear emotion (I've gotta get out of this fund!).  

More here: http://www.businessinsider.com/the-availability-bias-is-driving-investor-decisions-2012-10

So, as you set out on long-term investing (or continue your journey), keep your eyes fixed on the horizon and ignore volatility and the emotional tug or greed or fear. Acting on those emotions can be very expensive.