My current build:
http://downloads.2kgames.com/borderlands/website/skilltree/lilith/#21505050550500005005500
I found that build to be the best out of the two previous ones I had. The possibilities are high but in the end, after you get past level 40 or so I would say that Mind Games and Slayer are perhaps the most useful ones, and not only that but they are also pretty much essentials for me, I tried both previous builds without them both, and I did notice a noticeable difference, it wasn't pleasing, it made her generally more vulnerable.
The best thing to do, which I haven't done, but should have done at first, in my opinion anyway, is to go for Mind Games the fastest you can and max it. Then look for the rest. If not, go with Slayer first, head shots and critical shots in this game are not only essential but quite easy to do. Also, if you focus on only one gun type then Quicksilver is a must, especially for the SMGs, which is the gun type that Siren specializes in due to her Class Mods (the Mercenary variants) helping her for that specifically, SMGs in general already have decent fire rates, but combine a Double Anarchy SMG with a base fire rate of around 10.0 with SMG Proficiency levels along with Quicksilver and you'll completely destroy everything in your path without even the need of a single elemental effect.