How to tweak render when each render takes so long? Like any artistic project, until you can see that final render, it's impossible to know how close you are to achieving your goal. As we will all know, from being 'almost finished' to being finished can sometimes involve 20 or 30 tweaks. This is fine when we're designing in Gimp or PaintShop, but Blender is a vastly different animal and tweaking your initial rendered project, even 10 times, can mean 20hrs of rendering (and that's with a small 5sec animation). You could be looking at a week of rendering time alone on larger projects that you need to tweak multiple times. With Blender and all the options it has it seem highly unlikely you'll get all the parameters perfect first time many effects will be trial and error). But trial and error seems impossible if you want to create furiously. I couldn't wait 2 or 3 hours between each tweak... I'd end up on a murder rap!  

How do you girls and boys get around this problem. And it's not a matter of being careful and taking your time in the design phase. Even doing that... bveing as careful as you're able, it is just the creative way to look for improvements and want to speed this parameter up... scale that one down... change the effect... try another effect... try ten effects...
There are a number of ways to deal with this:


render a frame at low resolution and/or samples (i.e thumbnail). This
should reduce render time but still give you are. This can
effectively be done with the preview render in the 3d view.  
learn more about optimising cycles settings (e.g. tile size, bounces, clamp)
render out to a multilayer EXR and use the compositor to perform final
tweaks rather than re-rendering. 
for animation, Opengl playblasts
(the clapperboard icon in the 3d view) allow you a quick preview of
how your animation will look, but without full materials and
lighting. This can be very useful when normal playback is not at full   
bake background props (inlcuding lighting etc) to textures once you
are satisfied with them, that way you don't need to re-render them
again (see http://www.creativeshrimp.com/light_baking_tutorial.html for a discussion of some of these techniques).