You know you need plenty of reviews to keep your book climbing the popularity charts, but you also know that not all of them are going to be 5 stars-worth of praise and glowing recommendation that others read your book immediately. What can you do to help curtail the inevitable downward spiral of fear that might accompany a negative review?
Accept that you're going to get one eventually. Not everyone is going to like your book. What some people find riveting, others will find boring. All you need to be concerned with is reaching those readers who do like your work.
Consider your worst fears before you ever put your book 'out there', and then do your best to eliminate the ones you can. Things like grammar, spelling, and punctuation can be caught during the editing process, while allowing your friends and family to read the book before you publish it will give you some feedback on readability, syntax, character development and believability, and any plot holes. If you deal with the things that you can control, you will be less likely to get upset about those you can't.
Finally, remember to see each review as a chance to gain insight into what works and what doesn't with your work. If someone is simply stating a preference for another genre or style of writing, that's something you can let go of pretty easily. But if someone is commenting on the fact that there is something wrong with the writing, or that the plot doesn't work, take those things into consideration and evaluate your next piece more closely.
Take your book reviews in-stride, and whatever you do, do not respond to the bad ones. Remember to be thankful that the alternative that no one wants to read your book or comment on any part of it isn't true. Every review is welcome.