The basic reason you see your prints come out too dark is simple: your screen is too bright!

I’m not being silly here; that’s the solution to about 90% of the cases (assuming you know how to calibrate a monitor, that is.)

The iMac is a frequently cited culprit here: it’s not possible to get the luminescence into 80-90 CD.m2 range with the built in controls.

Here are a couple of  solutions: one costs $175 and up,  and one is free (or $10, if you want the “pro” version.)

For $175, there is Coloreyes Display Pro, at http://www.integrated-color.com, and for free there is Dark Adapted at http://www.aquiladigital.us/darkadapted/

I’ve used the latter, but not the former. (Instead, i use X-Rite’s ColorMunki; $500.)  Like ColorMunki, Coloreyes Display Pro is a software/hardware combination that you use to calibrate your monitor.

However, if you’re just trying to get on your print what you see on your screen, I’d certainly have no problem suggesting that you give the free version of Dark Adapted a try. It’s got a stellar reputation, lets you manipulate R,G and B individually, as well as setting the display level.

You can create presets (in the Pro version) that will let you instantly dim your screen for a realistic look at your image.

(I should note that my recommendation re: Dark Adapted is just for screen dimming, not calibration. If you’ve got a hardware calibrator, use it; then use DA simply to dim the screen so you can get a sense of the print luminosity. Setting DA to 70-80% is a good starting range.)

So: if your monitor setup allows it, you just need to dim-down your screen, into the  80-90 CD.m2 range. If your calibration hardware allows it, use that instead. Or, have a go with simple dimming using Dark Adapted.

More tips:

First, in Photoshop, go to your preferences, and select Interface. In the “General” box at the top, you’ll see “Standard screen mode” “Full screen with menus” and “Full Screen.” (These are the three backgrounds you cycle through when hitting the “F” key.) Select the popup menu for “Full Screen, Color” and choose “Select Custom Color…” and set it to pure white (255,255,255.) (Set  “Full Screen, Border” to none.) Exit prefs.

Now, when you’re looking at your photo, cycle to the white screen. This will give you a much better idea of the contrast and tonal range in your photo.

Second: get used to using the view/proof setup to look at your images before printing, using the paper simulations and ICC profiles you’ll eventually be printing on.

Combine this second tip with the white background from the first, and the proper monitor brightness, and there’s a 90% chance that your photos will no longer be “too dark.”