Full Color: Make sure the file itself and all embedded art is in CMYK mode. Embedded images need to be at least 300 dpi at 100% of the finished size. Convert all text to outlines. (With text tool, select all>Type>Create Outlines)
Spot Color: Usually used for jobs requiring only 2 colors [black & 1 other color, or 2 non-black colors] and colors defined by their PMS number. The trick here is to use your chosen colors consistently, not use 2 or 3 reds, for example, not just the PMS206C that you originally chose.
File Formats (in order of preference)
Illustrator (.ai) : Create pdf compatible files, no compression
Illustrator EPS (.eps)
PDF: High quality print, preserve Illustrator editing capabilities, no compression