United has issued a last-minute deal for its Explorer Card - sign up by January 8 to collect up to 65,000 bonus miles

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• There's only a little bit of time left to get the highest-ever public sign-up bonus on the main United Airlines credit card.
• The United Explorer Card offers up to 65,000 miles for new applicants: 40,000 United miles after spending $2,000 in the first three months, and another 25,000 miles after spending a total of $10,000 in the first six months.
• Usually, the card only offers 40,000 miles unless you're specially targeted for something higher.
• The bonus is enough for a round-trip flight to Europe, or even a ticket in United's new Polaris business class.
• The offer is only good until January 8; after that, it drops to a lower amount. If you have a use for United miles, now is the time to apply.

The best way to get a ton of frequent flyer miles quickly - aside from booking an incredibly expensive and long flight - is signing up for a new credit card and earning its sign-up bonus. This style of "travel hacking" is basically a way get flights or hotel fees for free; you can get a massive haul of points just for opening a new card and spending money that you were going to spend anyway.

The key is to make sure that when you open a card, you're getting the best offer that you can get. Some cards offer higher bonuses seasonally or during promotions, some products have higher bonuses when they launch, and some offer special bonuses for individually targeted customers.

If you tend to fly United Airlines and you don't have the United Explorer Card, now is an excellent time to apply for it.

For a limited time, United and Chase, which issues the card, are offering the highest-ever public offer on their co-branded credit card.

When you open a new card, you can earn up to 65,000 miles when you open a new card. The offer is broken into two chunks - you'll earn 40,000 United miles after spending $2,000 in the first three months, and another 25,000 miles after spending a total of $10,000 in the first six months.

This offer is only available until January 8, 2019, so don't wait.

While that spending requirement is higher than many cards, there are plenty of ways to meet it without spending more than you would otherwise. For this card, earning the full bonus would require an average of $1,670 per month for the first six months, which is certainly doable for a lot of people.

The United Explorer card was revamped this spring, and comes with some useful perks and benefits.

One highlight that sets it apart from the other airlines' mainstream cards: You'll get two United Club passes per year. These passes get you entry to United Club lounges, where you can enjoy comfortable seating, Wi-Fi, free food and drinks, and more before your flight. Normally, one-time entry to a United Club would cost $59 if you didn't have a membership.

The United Explorer card offers a free checked bag for you and a travel companion when you use the card to pay for your tickets. A checked bag costs $30 each way, so if you and another person on your reservation both take advantage of the free benefit, you'll save $120 on a round-trip.

Additionally, the card offers an application fee credit of up to $100 for Global Entry or TSA PreCheck - a solid value if you don't already have one or both of the services. While it's not a published benefit, United cardholders also get access to more saver award space than other United members, which makes it easier to find good flights when it's time to use your miles.

The United Explorer earns 2x miles per dollar spent at restaurants, hotels, and on all United purchase, as well as 1x mile on everything else. It has a $95 annual fee, which is waived the first year.

Should I apply?

This is the best public sign-up bonus we've seen on this card, so if you're eligible to earn it, now is the time to apply.

Keep in mind that you won't be able to earn the bonus if you currently have the United Explorer Card, or if you've earned a sign-up bonus for this card within the past 24 months. You also may not be able to get approved if you've opened five or more personal credit cards in the past 24 months.

Aside from that, if you tend to fly United, the Explorer card offers a solid value, and waives its annual fee for the first year, making the bonus purely profit.

Just keep in mind that we're focusing on the rewards and perks that makes this card a great option, not things like interest rates and late fees, which can far outweigh the value of any rewards.

When you're working to earn credit card rewards, it's important to practice financial discipline, like paying your balances off in full each month, making payments on time, and not spending more than you can afford to pay back. Basically, treat your credit card like a debit card.

Click here to learn more about the United Explorer Card from Insider Picks' partner: The Points Guy.