Soak your lasso in warm water for 10 minutes or so then scrub with saddle soap. This will remove any wax that’s helping to stiffen the leather as well as make your lasso pliable.
Rinse the lasso thoroughly and, while still damp, bend, twist, roll and crunch your lasso for another 10 minutes or so to work some flexibility into its bones.
Massage an oil into the leather of your stiff lasso, making sure to get in between any braiding or other ornate designs on the leather. Some folks recommend baby oil, others say olive oil and a few others insist hydrophane oil, sold at tack and feed shops, is the way to go.
Keep bending the lasso every chance you get and apply oil at least once a month until it’s baby face soft.
Use the lasso as much as possible. You are not going to soften a lasso by letting it rot on a hook in the barn. Even if you don’t use the lasso regularly, take it down to bend and twist at least once a day while you’re walking the fields or watching TV.