How to cut derailleur cable housing This is a job I will do very rarely, so I would prefer to avoid buying a specialty tool, if possible.

Unfortunately I don't own a dremel. Side cutters just mashed everything and did not cut at all. Are there any other methods you can recommend?

EDIT: I think this is different from the brake cable housing question, in that side cutters will get the job done with a little effort on brake cable. They have a much harder time cutting the parallel strands of derailleur cable housing. 

EDIT2: I asked the guys at the store where I bought the housing to cut it to length for me and they didn't mind. :)
I see you don't want to buy the proper tool - do consider that while some are silly expensive, there are also reasonably priced alternatives.  Brands like Birzman and X-Tools are a third the cost of a Park or Pedro's tool.

Another option is to use a rotary cutoff disk in a tool like a dremel, or even a full-sized cutoff wheel in a grinder.   If you have one already, then this is a no-cost.  To buy one would cost more than the right cable cutter tool as already suggested.