If you don't jump rope, you need to. Look around, any legit athlete jumps rope to build up speed, conditioning, and agility. I wonder if there is a professional boxer/fighter out there that doesn't train with a rope? Use it a method of warming up instead of the treadmill, incorporate it into a workout, use it as a cool down. However you fit it into your workout routine you need to be doing it. It took me about 6 months of casual practice to learn how to do double unders with confidence (two rotations of the rope for every one jump). Most people build up to hundreds of single jumps, then one leg, crossovers, double unders, then combinations of them all. Try it and have some fun. How long to do 250 singles? Max double unders? Can you do crossovers for 1 minute? What about when you are fresh versus tired? etc.
There are many ropes out there from the cheap old school beaded ropes (think kids playground) to the expensive Buddy Lee ropes ($30-$40ish) that are used and recommended. I started with a I made purchases from http://www.buyjumpropes.net and they offer various styles at good prices. I would recommend both the
Cool inspirational Video :Buddy Lee Power Jump Double-Under