For online shoppers, Amazon's Prime Day on July 11 has become as highly anticipated as Cyber Monday. For Amazon, it's been even more successful.

This year, the third annual Prime Day, deals start at 9 p.m. ET on July 10 and continues for 30 hours. There will be "hundreds of thousands of deals," according to the company.

With so many sales and and so little time, here are some of the most common mistakes to avoid when shopping Prime Day:

1. Paying $99 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 $99 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.

Because of this, 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."

But 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, have some of the best price cuts, as do tech and electronics.