It depends on what you are doing with the serial stream when you've got it and what control you have on the design of the receiving circuits.
cbarn24050's advice is ideal if you want maximum flexibility of the encoding technique and have no control on the receiver.
irstuff's solution is ideal if you are sending the data over long distance or are designing the receive circuit too.
Cypress semiconductor's hotlink device (CY7C923/33)is simple to use although designed for 160MHz upwards (It automatically pads the stream if the parallel rate is slower). There's some good apps notes on Cypress website too.