Just do what this guy does:
http://youtu.be/9Jrmu-caCFY
By the way, notice that he's got his leg over the saddle before he even comes into the picture.
Also, you shouldn't be landing on your butt. You should be "landing" on your inner thigh and sliding across. It should really be more like stepping across the saddle than jumping on.
In "The Complete Book of Cyclocross" Scott Mares describes it as being like putting your arm around a loved one. That's a great analogy and when you do it right you'll feel exactly what that means.