Dan R wrote:
It sounds like your monitor may need to be calibrated.
9 out of 10 times, this is the answer.
Right out of the box most computer monitors are way too bright to use for photo editing and adjustments. This causes you to make your prints too dark. They look fine on your screen, but not anywhere else (prints or online when viewed with a properly calibrated monitor).
They also are rarely very accurate rendering color, so will cause you to misadjust your image colors, too.
The solution is a calibration suite. That uses a "puck" that you position on the monitor to measure it's brightness and how it's rendering color. First you adjust the brightness to a more proper level, then the software runs a series of color sample tests to arrive at an accurate color profile, which is then applied to the monitor.
Monitors also gradually lose brightness and shift color rendition over time, as they age. So they need to be re-calibrated every so often. I have my Datacolor Spyder software set to remind me every 60 days... some people do it more often, others less often. 60 days works for me, but it's up to you to decide how often you want to take 15 minutes to re-calibrate.
If you do very much printing, a calibration device will save you money in wasted ink and paper (or the cost of having reprints done, if you send the work out to a printing service).
There are several different brands of calibration devices... I use a Datacolor Spyder suite (hardware puck and software) and it works well. But others seem to work fine too.
You can buy a lower cost, fairly simple calibration suite that only works on a computer screen... or one that can be used on CRTs, flat screen TVs of various types, projectors, etc. Some, like the one I use, can even read prints to help calibrate one's printer with custom profiles (used to "softproof" images prior to printing). I mostly use OEM inks and papers and printing processes that provide good profiles, so rarely need that feature. But it's there if I want to use it.