Relationships are hard work and sometimes long-term relationships require even more work, especially to keep the spark alive. 

According to relationship coach Christie Hoang, there's one simple rule that couples need to stick to, no matter how long they've been together, to ensure they stay together.

Hoang said if couples stop doing this one thing, they 'always' break up. 

Hoang recently shared a TikTok video of a woman offering some sage advice when it comes to relationship longevity, and as the founder of Men's Mental Health Institute, Hoang gave the tip her stamp of approval.

The number one rule is to always flirt with your partner, regardless of how long you’ve been together.

At first, the video suggested that the advice might be silly because it is “such a simple rule," but the proof remains; flirting can “reignite the spark in your relationship.”

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“You feel as though you drifted away from your partner; you don’t necessarily share the same intimate energy that you used to; you just no longer have that playful energy,” the expert stated.

Turns out, just because you're coupled up and committed doesn't mean you can relax when it comes to nurturing "the passion."  

There is a misconception that flirting is only used when trying to attract a potential partner.

Many couples stop flirting with each other once they settle into coupledom, and Hoang isn't the only expert to warn of the dangers of neglecting this simple tactic towards making things last. Sex coach Amy Levine told Bustle, "When we get to the comfortable, stable place of knowing we have 'secure' status with someone else, it's common to lose the playfulness that got you together in the first place." 

In the TikTok Hoang shared, losing that playfulness seems to be the death knell for couples that get complacent or comfortable in their committed status.

Sarah Hunter Murray Ph.D. explained in an article for Psychology Today, "Having our partner flirt with us is one of the ways we feel loved, special, attractive, and cared for. In contrast, the absence of flirting in a long-term relationship can make us doubt our partner's feelings and attraction towards us."

She further added that although often relegated to the early stages of dating, flirting in established relationships and marriages can have a positive impact on both partners and "may play a role in reducing fighting and increasing self-esteem."

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Having trouble flirting with your partner? There are some tips to help.

If it has been a while since you last flirted with somebody, don't stress. It's a lot like riding a bike. 

As clinical sexologist Shauna Harris explained, it can be as simple as sending a cheeky text to your spouse in the middle of the day or just taking the time to "laugh at their jokes."

Remember, flirting won't solve all the problems that come up in relationships, but it's an important part of desire and intimacy in a relationship. Reminding our partners that we still feel butterflies for them even though the daily schedule can sometimes get in the way can make all the difference in feeling connected. 

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Lauren Reams is a writer at YourTango who covers entertainment and news.