Have you done any sort of colour calibration on the screens of the computers your viewing the pictures on?
You can do this with a device such as a Spyder, Colormunki or several others. Basically it looks at the colours coming from your screen and works out if they are "correct" or not - if they aren't then it tells the computer how to make them correct.
For example, my laptop screen has quite a strong blue tint to it - I don't notice too much until I use my spyder to "fix" my screen but once I do it's very noticeable.
The problem is, that if your screen is not calibrated then you will see the colours wrong. For example, you might see a skin tone darker than they actually are. This will then cause you to correct the colours of your picture to be the colour you want. So - you'd lighten that skin tone to make it look correct.
When you then send that picture off to print, the printers will use the colour codes you send. Their printers should be correct, meaning that the colour you made look correct on your uncalibrated screen will print too light.
On top of this, different papers and inks will show colours slightly differently to each other and indeed different to a screen. For this problem, Lightroom 4 offers a mode called soft proofing. This allows you to select a paper / ink to simulate and it will try its hardest to show you how your picture will look once printed. This doesn't make as much as a difference as an uncalibrated screen, but it's still there.
So - if I've got a picture that I want absolutely right, I will therefore do the following:
1 - Calibrate my screen to make sure the colours are accurate
2 - Edit the master image inside lightroom to get the colours I want
3 - Create a proof copy of the picture for the required paper/ink
4 - Correct the colours of the proof copy to match the edited master image