when reloading,what sort of case preparation is necessary for previously fired brass,besides sizing? when is case trimming necessary?

Size, and trim to length when needed (see your reloading manual for case length). Clean primer pockets, clean cases before you size so you don't scratch the dies. HTH, Dutch.

For max accuracy:

Even brass from the same lot, when you load one to different velocities' the brass can "stretch out more" in the heavier loads.

It doesn't always happen, but rather than get yourself fustrated with poor accuacy from your reloads, the few thousands that they can be off, is well worth checking. It changes the amount of bullet pull, simply because there is more brass hanging on to the bullet.

I always check my brass for length after firing and if they are consistent, I don't trim until they reach max case length. If they are off at all, I'll trim them to the length of the shortest piece of fired brass. I do not cut to the trim to length as listed in the manual. Why trim shorter than you have to?

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Live Free! Madison, Jefferson and all the boys paid for it, and so did our very own fathers.

If you say it real fast, "sizing and trimming" sounds simple.

Here are (some of) the steps you only read about in the fine print:

Case inspection: Looking for cracked necks and insipient head separations with eyes that needed bifocals 3 years ago reminds you that you really should get some god#^%$ glasses. Needless to say, all of your cases should at least be of the same headstamp, even if they fail to match in other ways.

Lubrication: by finger, pad, or spray. Too much, and your shoulders will be dented. Too little, and a case will stick in the die and you're @$%#&^ screwed. And don't forget the insides of the necks.

Resizing: Full length, partial, or neck? And don't even attempt resizing without the gun for which you're loading present so you can check for ease of chambering.

Getting the gunk off of the resized brass: Wipe it off with a cloth or paper towel, and don't forget the insides of the necks. Actually, this is so ineffecient and such a pain that most serious reloaders chunk the whole batch of sized brass into a vibratory tumbler for a few hours just to get rid of the lubricant residue.

Cleaning the primer pockets. Various devices are made for this thankless task. (Thankless because a lot of reloaders skip it, causing other reloaders to recoil in horror at the tiny pieces of diamond-hard primer residue they envision rocketing down the bore, eating the riflings for lunch.) By the way, if you did clean your cases with a tumbler, you have to inspect the flash holes now, because about one out of five will have a piece of corncob lodged in it which you'll have to dislodge with a tiny sharp object (some reloaders can use thier heads for this task). Some of us can perhaps relate to having a corn cob lodged in our flash holes.

Trimming: Actually, most brass doesn't "grow" all that fast, so trimming, depending on caliber and a lot of other variables, may not be necessary before several firings of the brass, by which time you've probably lost the brass in tall weeds or some such. Also, most chambers are pretty sloppy and a bit on the long side, so you nearly never need to trim with most cases until the brass has exceeded "maximum" length by several one-thousandths. Neck chamfering also effectively reduces case length, so if, as you should, you chamfer the necks, the need for trimming will be reduced. Which brings us to

Neck Chamfering: What this does to your fingers makes carpel-tunnel syndrome pale in comparison. Find a tool you can chuck in a drill or use one of the newer motorized systems to bevel the inside of the case necks. With most calibers, if you try to seat a bullet without this step, you shave nasty little strips of gilding metal from the bullet as it's seated. Now that the inside of the neck is chamfered, you might as well finish the job by turning the tool around and smoothing the outside of the neck.

By the time you've done all of this, the brass is now tarnished again, so back to the tumbler. After picking the corn cobs out of the flasholes again (maybe you should just do this after the second time you tumble, come to think of it) prime the cases with a "feel" type priming tool, so that you can throw those away which don't provide a tight enough fit as evidenced by seating resistance.

Now, I've purposely skipped weight-sorting the brass, gauging the flash holes, mic-ing the heads, pre-loading concentricity (run out) gauging, and a few of the other arcane practices that some reloaders substitute for religion. Just wanted to give you a taste that doing reloading right is not quick or simple. Have fun!