always keep two images of each (1 & 2), native rgb file untouched, original file size you have (1), duplicate it change to cmyk and alter dpi if its too low so you are not dealing with enormous sizes (cm) in in-design (2)
if you do have to re-size image make sure all three boxes in image size box are linked so all you are doing is altering size in cm not quality or file size
need to alter it - new layer, hide one beneath (keep original on base of that file but hidden), contrast brightness new layer, hide one beneath, name layers to suit (brightness for eg), top layer is the one you need but have access to all others
do not flatten type onto it if you have type in file, keep them on their own layers, when happy with it raster type and name that layer font name, don't want in-design telling you a font is not there when it could be hidden and not rastered
if you need one etched out and one not, just create new file for etched and name '**** etched', also i find if i etch an image - create base layer with a contrasting colour which is hidden in final file but there for checking, green for instance, that way you can see how good your etching is, 1 pixel feather can help, it helps make it look more natural
hidden layers are not affected by any work on levels, for eg, that you apply, only applies to ones visible and selected, do not work on file with original layers visible
many versions of the same image can create problems later, and if anyone else needs it they will get the correct one which can be an enormous problems in a large busy working environment, if you do need a few versions name files accordingly
Converting your vectors (illustrator files) to bitmaps (TIFF, PSD, JPG, etc) is good if your vector images are complex and have lots of nodes and transparency, it's not 100% necessary all the time but it does make life easier especially if you or your printer are using older equipment.
Vector files can also be a pain when the files go through a RIP (raster image processor), bitmaps just make the process much easier as anything being printed by a CMYK process needs to be rasterised (bitmapped) to make separations for the printing plates.
That's really good advice. I also find the separations preview is good too (think it is in the same pallette) particularly if you are working with rich blacks or spot colours. It's also good if you are working in newsprint and need to check your ink limits.