For online shoppers, Amazon's Prime Day on July 16 has become as highly anticipated as Cyber Monday. For Amazon, it's been even more successful.

This year, the sales start at 3 PM Eastern time and continue for 36 hours until Tuesday night, making for the longest and most globally accessible Prime "day" yet. There will be "more than one million deals exclusively for Prime members," according to the company.

Jeff Bezos's e-commerce giant continues to dominate online retail: Amazon is expected to take nearly half of the U.S. e-commerce market by the end of the year, according to a recent report by analysts at eMarketer.

To get the most out of the sales event, here are some common mistakes to avoid when shopping on Prime Day.

1. Paying $119 for Prime to participate

Yes, Prime Day is only for Amazon subscribers, but if you're not one, don't think you have to pay the $119 yearly fee to take part. New subscribers can sign up for a 30-day free trial and get access to the deals.

But remember to read the fine print. At the end of the trial, Amazon will automatically upgrade you to a paid membership plan. You have to affirmatively turn off the automatic renewal before the end of the trial if you don't want to get charged.

2. Forgetting to check your Amazon account beforehand

Make sure your credit card info, account and shipping info are up to date. With limited sales, time is of the essence. Don't discover you have to update your expired credit card in the middle of a purchase.

3. Falling down the Prime Day rabbit hole

"Part of the challenge on Prime Day is that are are so many products that can be discounted," says Rick Broida, CNET's online deal expert and author of blog The Cheapskate.

For that reason, online shopping expert Michelle Madhok suggests having a game plan that focuses on only what you need. Or, she advises, "sometimes the best thing you can do is go for a big ticket item."

Even if you drill down toward something specific, the number of products is huge, says Broida, so download the Amazon app and enable alerts. With deals posted every five minutes, it's the best way to keep track.

4. Assuming Prime Day has the best prices

"Not every deal that you are going to see is going to be the best deal in the history of deals," Broida tells CNBC. Amazon products, like Echo Dot and Alexa, will see some of the deepest price cuts, as will tech and electronics.

To know if you're really getting a great deal, Broida suggests an online tool "amusingly called camelcamelcamel.com. You can copy and paste the link from Amazon and see the price history for that product."