Add color to your scene that gives it a futuristic feel. If it is a space scene you can use deep blues, and if it is a scene on another planet you can give the sky a red or greenish hue. Just about any color can be futuristic if used in an unexpected way or when given an unusual intensity.
Create unexpected texture to elements of the picture. Make a night sky futuristic by adding swirls of black and silver, copper or deep blue. Create a landscape that could only be found on another planet or another time by making it unseemingly smooth or unbelievably course, like nothing seen on earth.
Use lighting to give a futuristic aura to the picture. Use natural elements to project the light in the picture, like stars, moons or suns. Contrast adds to the intrigue when you add sunlight with a visible moon, include two suns or backlight several unexpected planets. Indoor scenes can also have unusual lighting which can create a somber or cheerful tone.
Add objects that are not from the present. These can range from small details in interior scenes to larger, more obvious objects in larger outdoor scenes. Include building designs that are not seen in present times. Features such as spacecraft or unique clothing on the individuals in your scenes will highlight that the scene is set in the future.
Review your picture when you're done and have a friend critique it as well. Make a list of the things you've added which you feel have given it a futuristic feel. Ask your friend to do the same. Also, try to spot anything that detracts from the futuristic feel and get rid of it.