My experience with serial data has shown that it doesn't go very far on its own (about 25m). My suggestion, which I have used many times in the past is to use a serial to RS485 coverter at both ends of the line. One is used to convert the RS232 signal to a RS485 signal, and the second to convert it back at the other end. These converters are relatively cheap, robust, and only use one wire pair. They can be configured to work both ways along the line. I have never had any interference using them, even when using a pair within a multicore cable setup. I have used these setups for cable runs of over 400m.
A brochure for the converter that I use can be found at http://www.alfatron.com.au/brochures/a450_brochure.PDF
I am sure that others have their own way of dealing with these issues, however I hope this helps.
Regards,
George.