This is a great question! Walking into a bookstore without a particular book in mind is one of my favorite things to do. I particularly love going into used bookstores because you know at least one person has actually read the book before you.

One thing that I love to do is to go to a section of the store that I don't normally go to or that I would not automatically think of as the perfect section.
Then I scroll through the titles. If the title doesn't catch my attention then there is no chance for that book.

I move from section to section fairly quickly - if there is not a title or theme that catches my attention right away I move on.

The next thing I do is to read the back of the book ( this is always biased but it gives me a good idea if I want to rule the book out ).

Then I will move to the next section and go through this process again until I have several books to choose from.

After I have a few to choose from I check out the first chapter and flip through the book to see if it looks like a good read or not.

If I am torn between a couple of books I will often check the price and that will weigh heavily on the final decision.

One thing I always do is to bring a small notebook with me to write down the books that really catch my eye but are in the "wait till they go on sale" column or just in the "remember for next time" column.

When I finally come to my decision I take the books from the not now pile and put them back in their original spots. It just seems like the right thing to do and it gives me a chance to take one last look at the section.

This process usually takes about an hour. It is a great way to spend a Saturday or Sunday afternoon.

Have fun!

The two books I always look for in any bookstore ( because I keep giving them away and never getting them back ) are:

A Confederacy of Dunces by John Kennedy Toole
and
Stickeen The Story of a Dog
by John Muir ( not the childrens edition ).

Somebody once suggested the 5 page system. You flip to 5 random pages and read. If you don't want to put the book down you buy the book.

Seems to work pretty well for me.

I go find "A Practical Guide By Remembering Your Past And Future Lives (Even If You Don't Believe)". If there's a copy, I buy it because of the interesting content and the very attractive cover. I believe everyone should do the same...

Now... back from the spam...

I know I have a general interest, so I go to those sections and then start looking at titles. If one grabs me, or my intuition guides me there, I take a look, read the back blurb and then peruse the table of contents. I throw in bonus points if there are pictures...

Given that I'm blessed/cursed with an interest in almost everything, I usually walk in thinking of some category - science fiction, gardening, whatever - and head for that section. If some title, author or forgotten interest stops me on the way, I check it out. Usually, that means checking the table of contents, the first couple of page for writing style and level, and maybe the index, if non-fiction.
If it's fiction, interesting and doesn't appear formulaic, it's mine.
If it's non-fiction, not more expensive than my house, adds to my knowledge and is well documented, it's mine. Now, the hard part. Getting out of the store without multiple books.

When I go into a bookstore to choose a book I first start with a genre, mystery, thriller, travel, historical fiction, etc. The next thing I look for is an author I have read and liked before. Lately I am finding that I have read most of the books that my favorite writers have printed so then I start looking at cover summaries and blurbs to see what piques my interest.

This is not a perfect system. I have ended up with a couple yawners but for the most part it works well for me.

I am a sucker for the plain , not the colorful coated candies

I'm cheap, so if i go into a big chain bookstore i always start with the "value" books. Then in all honesty i start looking at covers and titles.

i know, i know never judge a book by it's cover but if i didn't have a particular book in mind this can actually tell me a lot about a book. If i come across a bunch of books all pink and white with curly-cue writing on them, i know i am getting a fluff read. regardless of the old adage, covers can actually give away the "feeling" of a book (i feel so anyway). So depending on my mood this can help me.