I work in the repro industry and most of my experience is with flexo these days.
The quality varies greatly between printers from dreadful to close to gravure quality. My aim to the reach the latter, but both repro technicians and printers have their role to play. It is often difficult to persuade printers the importance of investing in finer anilox cylinders and the routine cleaning of equipment, keeping the process ink set pure, establishing procedures including running to set ink densities, viewing proofs and press samples in controlled lighting (D50) and sticking to a well chosen process ink set, etc etc.
Colour management is also an important issue, ideally fingerprint charts should be printed at the target printers and profiles made, or at least effort should be made to obtain an icc profile from the printer, that describes their press colour space. This can be used in Photoshop for soft proofing and for inkjet proofing, this is a topic in it's own right.
From a repro aspect, this is my game, but I can only give you some important guidelines, experience is the great teacher.
1) Try to keep image colours clean, i.e. remove contaminating colours, if you want to keep greens bright then try to eliminate the magenta from that area and move the magenta tone into the black channel, similarly with oranges, cut out the cyan and move to the black channel. You need to use skill to rebalance the colour.
2) Remember not to allow colours to break to 0% within a smooth object, always have a minimum dot to travel across the image in all the relevant process colours, only breaking to 0% at sharp contrast points such as the edge of a leaf or the contour of an item of clothing. For vignettes, run from a chosen tone, e.g. 80% to a minimum tone, e.g. 2% and allow the minimum tone to travel to a suitable point in the design where the resulting edge is best disguised, stopping a vignette half way down a panel will result in a obvious and nasty printed result.
3) Make sure to keep a minimum tone value (possibly 2%) wherever you want to have tone, eliminate all tone below this from your image, make it 0%!
4) Remove all 'scum dot' areas from images, i.e. there should be no random dots on the image, only tonal islands of colour on each separation, in fact it is not out of order to run minimum tone right across the image in all used process colours.
5) Try to keep the amount of process colours used in any one area to no more than three, maybe consider a tri colour separation.
6) Flexo's main issue is the amount of dot gain which occurs at the bottom end, a 1% tone in an image can grow to 10%, or heaven forbid even 20%! although this is becoming more rare these days. Magenta is often the one which dominates and can cause the most problems, it can help just to drag down the tones in Photoshop in the bottom 1%-20% area.
Finally, just to pick up on Stephen's comments, I have been heavily involved in the Kodak NX plate system for a couple of years now, and I can confirm that it is a great product which when used with Digicap leaves the competition standing. However it comes at a higher price, though now it has been released to the wider industry the price may sharpen.
With the greatest respect to Stephen's comment, the point I would like to make here is that contrary to Kodak's claims, in my experience NX does not eliminate the need for complex repro procedures; the results can be superb if the repro is skilfully handled. You will forgive me if I don't expand on this here, I'm sure you can imagine that this is commercially sensitive right now.
Good luck
