The most important step in choosing the right publications is to figure out where your best prospects look for information on what you sell. 

A good prospect is someone who has an identified need for what you sell and has the desire and money to buy it. People who fit this description should be targeted first.  Where do your best prospects look when they want to buy the kind of product you sell?  These will be important "search corridor media" for you. 

So instead of publications with wide, general readerships, choose those that are (or contain) a search corridor frequented by your best prospects.