The in-print portion of an out of print and collectible book business is the speculative portion. If you can discover the new Harry Potter before everyone else, guess correctly on the next Caledecott or Newbery winner, or develop a mad passion for a new author well before the public discovers them-there is the potential to realize a large profit. Even though you are making a guess, the trick is to make it an educated guess, or you will be paying at or near full retail for stock which may never sell for retail again.
Routinely peruse Publisher's Weekly magazine and see what books are going to be published, (or as is the case with many children's titles, re-published). You can get a good feeling not only for trends but also how much marketing money/buzz a publisher is putting into a title or an author.
Also, for children's books, The Horn Book Review is a great way to identify interesting titles or upcoming authors and illustrators. A dealer specializing in in-print titles, no matter what the genre, should be studying both these publications as well as individual publisher's catalogs. Even for an out of print dealer, these magazines can help you determine which titles are generating buzz, and also ones are going to have a large initial print run and therefore would be wiser, to wait a bit and not pay full retail price.
Catalogs are great, but display is better, so I try to get to the in-print bookstores, chain and independent, at least 2 to 3 times a month. I'm also lucky enough to live close to a remainder warehouse, which I try to visit at least once a month, so I can also see which titles are on the way out of print. In some cases I'm able to purchase books I'm interested in, at a decent price, BEFORE they are attacked by the dreaded black marker.
To be totally honest, some of these recommendations are things that I SHOULD routinely be doing and don't; in part because I don't have the manpower, time, organization, or discipline.