Softening Denim Fabric ?

I recently bought a pair of black denim jeans.  I washed them according to the instruction and they came out even more stiff.    I don't have any fabric softener.   I wonder if you can recommend anything to try.   I have looked on Google but it was not very helpful.   
I did take them back to the shop where I bought them, and the Manageress, said hard luck, you have washed them so we cannot take them back, and we never have complaints about them anyway.    No wonder the Company is having a closing down sale.    Thanks  Mrs Bouquet

could try baking soda

Just get some conditioner that should do the trick

My darling MB. Have you ever tried using a mix of mashed soapwort leaves with a mixture of oil-du-mer? That was Growster's advice with one of his "specially made for Goosey" tablets twice a day. I only tried them once and thank god for that! Perhaps putting on the DVD of the Wallace and Gromit's "The Wrong Trousers" might just make you consider forgetting denims. You can get some soft leggings that will enhance your legs and, if you go for that, I'll be there for whatever dance you want to do.