Whenever I want my towels to get soft and fluffy again I throw in a tennis ball or you can use a dryer ball as well.
The tennis ball works well for me.
Also I find that since I started using Vinegar instead of Fabric Softener my Towels stay soft and fluffy longer.
Another thing to avoid your Towels from becoming stiff is to avoid putting too much detergent in the washing machine when washing the towels as too much detergent can cause soapy residue to build up on the towels and the washing machine won't be able to remove all the soapy residue making your towels stiff.