Improve Air Circulation
Air circulation is the key to cooler temperatures. In the attic, for example, insufficient venting can produce scorchingly hot temperatures as the roof radiates the heat it absorbs from the sun into the space. The same principle applies to a porch -- if it opens to the outside air and is well-ventilated, it will remain cooler. You should be able to open all the windows, and they should have screens so you can leave them open without the danger of your porch attracting a bug convention. A strategically placed fan may improve circulation, and it will also help with insect control.
Lighten the Roof
Dark surfaces absorb ultraviolet sunlight and radiate the increased energy in the form of heat. This effect is especially important in the roof, which receives the most direct sunlight of any other part of the porch. According to Energy Star, replacing dark-colored roofing with reflective materials can lower the roof's surface temperature by as much as 100 degrees Fahrenheit. You don't even have to replace the roof. Simply coating your existing shingles or rolled roofing with white elastomeric paint will counter heat gain in the roof. A number of structural and design considerations will govern the decision concerning the best way to treat a given roof -- a professional consultation may be in order.
Use Light Colors Outside, Dark Inside
The principle of reflectivity also applies to the outside walls, posts and trim of the porch; the lighter they are, the less light they'll absorb and the cooler they'll remain. Inside the porch, however, it's a different matter. Darker surfaces absorb incident light and pass it through the walls to the outside. The same is true for the floor. A bright floor reflects incident sunlight, creating more energy for other surfaces to absorb. A dark floor, on the other hand, absorbs the incident light and radiates heat down into the crawlspace. If the crawlspace is properly ventilated, the heat will dissipate.
Don't Let the Sun Shine In
The people who built stately old Southern homes always provided plenty of shade for the porches in the form of window overhangs and shade trees. You may not have time to wait for a shade tree to grow -- there may not even be room for one -- but you can provide overhangs in the form of inexpensive awnings. If the porch has windows, the awnings will prevent sunlight from shining through them and turning the porch into a greenhouse. In the absence of awnings, hanging sunscreens in the windows will have the same effect.