Never ripen tomatoes on a windowsill.  The direct sunlight will cause them to ripen unevenly.  Instead, put under ripe tomatoes in a brown paper bag with and apple or pear.  The fruit gives off natural ethylene gas that speeds up the ripening process.

Store ripe tomatoes at room temperature.  If it's extremely hot, the butter compartment of the refrigerator is ideal.

Most people slice tomato horizontally, but if sliced vertically (from stem end to bottom), they'll retain more juice.