1/2" is a little thin for a table top, especially without an apron but I am going to answer your question anyway.
Woodworking 101
Screw the top on. Use washers, and oversized screw holes. Slot the screw holes perpendicular to the grain for wider tops.
Back off the screws slightly when snug so that they are secure, but not tight.
This will keep the top flat relative to the frame while allowing for expansion and contraction.
There are certainly fancier ways to achieve this, but I will leave you to discover those. Hint: You need to achieve two things. Allow for expansion contraction, while keeping the top flat relative to the frame. You meet these criteria you are in good shape.
Remember, you can't stop wood movement, but you can guide it. Make sure your table base is designed to be stout enough to keep the top flat. This is one of the functions of a traditional table base with aprons, other than supporting the top.
You might not want to hear this, but a 1/2" thick piece of Birdseye maple 14" wide is not going to want to stay particularly flat on its own.
Also. It is important to understand what is going on, with wood movement. That is the key to learning how to deal with it. Do a search on wood web. Learn everything about moisture content, drying, and wood movement in general that you can.
It is also a good idea to Use a wood movement calculator such as this.
http://www.woodweb.com/cgi-bin/calculators/calc.pl?calculator=shrinkage
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P>If you have an iphone, there are several good apps as well.
I would suggest a rule of thumb for movement but it would be better if you learn for yourself, what percentage of moisture change you will likely see in your region, plug in what species you actually work with, and come up with your own numbers.
Good luck.