Avoid excessive use of black.
In general you should avoid large graphics or logos and refrain from using large font sizes, bold settings and underlining
REMEMBER: Color faxes contain more data and will take longer to transmit.
Plan your white space.
Faxes are transmitted as a collection of horizontal lines. Even one small bit of black on a horizontal line requires that the whole line be transmitted and printed. When finalizing your fax piece try to create your document in blocks leaving vertical blank space in between and avoid vertical lines or frames around the text