There was a previous post about using baking soda and white vinegar.   You might also try using a cup of baking soda once a week or biweekly and follow it down with three or four cups of boiling water.  The water will make the baking soda more alkaline and be more effective.

While that may help, there is another thing to try which is fairly inexpensive, "Washing Soda".  Note, this is for slow not plugged drains. It is typically found in supermarkets and can be used to help extend your laundry soap.  If you have a problem finding it, you can call 1-800-524-1328 and ask to locate a business that carries washing soda.  They were very friendly when I called!

Washing Soda is a caustic (pH of 11), so you need to be careful with pipes and not over use the soda. Some say not use use with PVC drains.  

Use around 1/4 cup to a 1/2 of cup of washing soda and the flush it down with some boiling hot water.