It all depends on what you are trying to achieve, I use a layer pass render technique. Its what all the major animation studios uses too. You render out almost everything out in a separate layer and composite it in a third party program, Like Combustion (which is legal). This way you can achieve soft shadows, motion blur, distance blur, High quality Global Illumination on the backgrounds, and blurred reflections. Its by far the best way to render because then you can tweak your footage and get instance feedback without waiting 20 or so minutes for a result. The problem with this tatic is that you have to know what you doing when you composite, it has a little learning curve to it. (as with any other thing in 3ds max)
Here is a tutorial to give you more insight on the subject.
Rendering in layers
I highly recommend you learn this way of rendering. It saves you tons of time in the rendering process.
Now what if you don't have a third party program or dont have the time to learn the program and learn layer rendering? Then you really need to look at your max file. Do you have more lights in your scene than what there needs to be? Are there some objects that have ray trace materials on them that really dont need to show reflections. (example: a highly polished wood furniture in the background) You need to go through your file and see stuff that is slowing you down. I would recommend sticking to mental ray. Scanline tends to be really slow. And your test renders should not involve complex lighting calculations. They are only to render out the movement so you don't need to see all the reflections and soft shadows.
Now after you've gone through your file and fixed everything there are one of two things that you need to do. You need to sit down and ask yourself, "the way this render time is going, do I have enough time before the deadline to complete rendering?" If the answer is yes, then you are good. If the answer points to No here are your two options.
1: Make sacrifices, first thing to go should be motion blur. Then distance blur, and then soft shadows. Get rid of objects that are causing render time to increase. Its very painful but sometimes its what has to be done.
2: Get more computers. Have a Render Weekend and gather as many computers that you can and have them all render a piece of the animation. You have to load 3ds max on each of them and set the paths to the textures and all. Or if you know networking computers you can run backburner, which comes with 3ds max.