Colour space : CMYK
Vector where possible.
Minimum 300DPI resolution for Raster Images.
File Format: PDF
Make sure your artwork is in CMYK colour space.
Open your printer and you'll see it contains Cyan, Magenta, Yellow, blacK ink/toner.
Artwork needs to reflect the physical capabilities of your machine.
See CMYK vs RGB Here
Low quality artwork results in low quality prints.
Find your image DPI Here
Not everybody has your fonts installed.
If you are sending artwork somewhere else without the fonts outlined, you'll cause frustration and delays.
If you've received artwork without the fonts outlined, do this Here
File & Artwork Format
PDF is king.
This one-size-fits-all format will store raster, vector, colourspace, clipping masks and basically everything your printer needs to produce the results you expect.
Sharpen up your artwork and vector where possible, see Here