Why are you donating your time? By the looks of things, you're quite talented and can charge quite a bit. Unless money isn't your motivation, which is entirely plausible.
The freelance design business is great right now, sometimes I get work on interesting projects, but rarely in the startup world. Thats probably the most interesting space to be doing UX right now hence the post.
I have also done this before for Simple Invoices: http://www.jasonstrachan.com/?p=290