A commonly reported calibration issue we hear at the lab is that the prints received look darker than the images on your monitor. This is usually related to a monitor, many of which are brighter than prints can be because of the backlit nature of most screens. Simply lowering the brightness on your monitor can correct most of these issues. Alternatively, you can purchase a color calibration kit, such as X-Rite ColorMunki, which will tell you how much to adjust the brightness.
Mismatched Color Profile
All UnitPrints professional printers and lab machines used the sRGB color space for printing photographs. Please ensure all files submitted to UnitPrints are in this color space, and have this color profile attached.
You can ensure files are appropriately profiled, by making some small adjustments in Photoshop. To edit color settings in Photoshop, click "Edit" -> "Color Settings". Be sure to set "RGB Working Space" to "sRGB IEC61966-2.1". This will set Photoshop to work with files in sRGB. However, this alone does not convert your images to sRGB.
Individual images you are working with, will need to be assigned sRGB. To do this, click "Edit" -> "Assign Profile". Then select sRGB (from working profile, or the profile list) and click "Ok".
Photoshop can warn you when these settings are not correct, and correct them. Simply click "Edit" -> "Color Settings" and check the boxes labeled "Profile Mismatches" and "Missing Profiles". This will display a warning anytime you are editing a non-sRGB image, and offer to convert it for you. The Color Management Policies option will also allow Photoshop to convert images for you.