I have the occasional unlucky month, but generally I'm pretty satisfied.
If a particular weekend off is very important to me, I bid for that and nothing else. On the other hand, if I don't need any specific days off, I treat Carmen as a luxury and bid heavily for a style of tour or even one tour in particular. This works well for me.
However, it's when I try to mix days off with a 'lifestyle'-type bid that I have come unstuck.