If you do extensive editing of your photographs that might result in a colour change (e.g. toning, selective colour replacement, B&W etc.) then it's really a simple two step process:
1.
Colour-calibrate your monitor: use either a hardware device such as an EyeOne, Spyder, Huey etc. or visually calibrate using Adobe Gamma or other software such as nVidia's Display Optimisation Wizard.
2. When saving your finished image for output to either an online or drop-in print service, ensure you observe the following:
- Your image depth is 8-bits per pixel
- Your image mode is RGB colour
- Your image colour space/profile is sRGB IEC61966* and/or
- Your editing software allows the profile to be embedded in the image*
Every print service can handle sRGB images since they will map the sRGB data to their own output space/profile; the only issues that you may encounter is when there isn't sufficient gamut (colour range) at the output stage that will match 100% what you see on screen - this is not uncommon since the variable is the printing technology used by the particular vendor and, accordingly, this is beyond your control.
*this point is particularly important when it comes to print services since, if the profile is not embedded, the majority of print services will
assume that the file is sRGB. Where this causes problems is if your original file used a different colour space/profile than sRGB - resulting in washed out colours (Adobe > sRGB mismatch) or strange blue/green shifts (ProPhoto > sRGB mismatch)
Some professional print services will accept up to 16-bit per pixel files in other colour spaces and will even provide you with the ICC profiles of their output devices so you can convert your image to match. Just call up your intended service provider and ask them how to best colour manage your files for optimal results on their printers.
Colour management and theory can get complicated, especially at the levels where you need to ensure that the trifecta of camera / screen / printer all produce consistently accurate results across the board... but for less critical consumer use, the information outlined above should prove adequate.