Yep throw them in the washing machine, or soak them in some warm/hot soapy water. They're not going to get any softer if they're full of dirt/sweat etc! Just whack some oil on the leather afterwards.
As for leather reins, if you buy new ones, I like to chuck them in a bucket of oil and let them soak in the sun for a few hours. Put some oil in the bottom of a bucket, put the reins in, swish them around so they'er completely wet with oil, and then leave them in the sun/somewhere warm. After a few hours, take them out, polish them off with a soft cloth and store then in a cotton bag.