prepare as in get ready for paint? cleaning? for cleaning, wash in soap and water, then maybe a light spray with windex or isopropyl alcohol, a layer of primer and then paint.
If you're talking sanding and / or puttying, it's a little more complex. Sanding you should do with fine grits, I use (in sequence) a 400, 800, 1000 and finally a 2000 grit autobody paper. None of these are gritty enough to really remove much detail, and what is 'reduced' in terms of detail can be put back in with a scriber / backside of a No. 11 knife blade / drill bit in a pin vice / tip of a No. 11 knife blade.
Puttying is trickier, but you can 'gate' the putty, depending on the type you're using. For example, Tamiya's putty, I tape off all around the area I want to fill, only leaving exposed that area for the putty. This way, I preserve the detail and have less clean up to do. Then, remove the tape, reinstall very slightly further away from the filled area, and then sand as above, fill in detail as above.
I've recently discovered Vallejo's plastic putty, great stuff, and from what I've read I think it's similar to Mr. Surfacer (though I've never used Mr Surfacer). It's only meant for very minor gaps / holes / scratches, but you apply, allow to dry, wipe the excess with a Q tip soaked in isopropyl alcohol. No sanding / remedial work required.
I hope I've understood what you meant by prepare the body, but this is all I can think of based on your question.