If you are using the internal renderer, here are some tips for speeding up your render times.

1. AO:
        AO is just a simulated GI process. It looks really good to use for your final render, but while doing test renders, definitely turn this off. When you are ready to start perfecting your final render, turn AO back on and reduce the number of samples in your area light settings as well as AO sample settings. Set all these to 1 till you feel you have the right amount of light in the right areas, then raise them a little. For final render, I recommend around 9 samples for all.
2. OSA
       While OSA is a definite for final renders, turning this off for your test renders will speed them up quite a bit. Even when on, setting the samples to the lowest setting (5) is enough for some scenes. Another way to speed things up (concering OSA) is to render your image at double size or larger, with OSA turned off, then scaling the images down later in Photoshop or some other 2D app. This will give the same effect as OSA, but can be faster on certain types of scenes. But for test renders, just turn this off.
3. Area Lights
        Using area lights with AO and OSA? This is the slowest you could possibly render anything. While it looks very nice for your final render, try turning off the ray shading button of your area lights for quicker test renders.
4. Ray Tracing
        the Raytrace button is the quickest and easiest way to add instant realism to your scene. While its possible to fake the effects of ratracing in your scenes (animators rarely use ratracing, especially not whole scenes), it can take alot of trial and error to get somthing looking just right without it. I recommend using it for rendering still scenes, but if you are venturing into animations, turning this on can turn hours and hours of rendering into days and days. For early test renderings of your scene, you can turn this off to speed things up.
5. Subsurfacing
        In the modifier panel for subsurface, there are settings Levels: and Render Levels: You of course know that Levels: represents the level of subsurfacing you see in your 3d view, and Render Levels: represents the level of subsurfacing that will be done at rendertime. Reducing this level for test renders can speed up rendertimes for complex scenes.
6. Render Size
       Also,(as you have already tried) reducing the size of the image to be rendered will speed up render times. I recommend setting the X and Y to constant settings, and using the quick buttons in the render panel to change the size for test renders. Hitting the 50% button will render an 800x600 scene at 400x300, and the 25% button would render it at 200x100. Its just easier : )
7. threads
       Enable the use of threaded rendering by clicking the threads button in the output panel. This will render two sections of your image at the same time, and even on a single core computer with no hyper-threading at all, I have noticed a slight increase in speed. On your system, it should show a little more increase.

With all these things in mind, you should be able to render with Blender's built-in renderer very quickly, and adjust the different factors as needed as you approach the final stages of scene creation.
While Yafray has the power to render some very realistic images, I can not reccommend using it at all if you are interested in speeding up your rendertimes. At its most basic settings, it is still takes about twice as long to render than the built-in renderer(on Windows systems)