(Let me preface this by saying I wrote this while the last few comments were made. Luckily I copied before trying to submit, because I had to log in again. :) )
Here are some suggestions/thoughts and things I like to know when designing for a client.
From what I gathered from reading the brief and doing some internet searches, you do public relations and advertise your clients and/or their product (books, music, bands. talent etc.)
If you need the signature (and I am going to use the word signature because a signature says more about who you are and is more brand related) to say more about what you do, I suggest you need to go with a more literal rather than abstract symbol (not a wordmark, not just a logotype). But, considering what you do and what your selling (people and/or their product), it will be a little difficult to do literal symbology. This takes us back to the abstract.
Thus, you may want to consider adding a tagline. Maybe something like "Promoting people and their products". Think about Enron. Their name says nothing...well, these days it says corruption...but the name itself says nothing about what they do.
Also, find some signatures that you like the look of, go ALONG with your branding sliders, what is it about them that you like and mention them in your brief.
Other things I like to know when working with a client. Where are you located at? Are your clients local, regional or national? From what I gathered you are in Houston. And the reason I ask this is that maybe there is a local landmark that can be incorporated into your signature. If you are national, this really does not apply.
Who is your competition? What are their websites? (for research) What differentiates you from them? If we are going to talk branding then we have to talk differentiation!
Now, let's be honest and some on here are going to very much disagree with me. But you get what you pay for. The company I used to work for charged $10,000-$15,000 for a signature and that did not include all the logo standards that should come with it. So, you may want to raise the contest amount, which will definitely get you more designers participating. I am not saying the entries will be any more conceptually thought out, but it will get you more designs. Also, you can extend the ending date. As one designer already pointed out, you have a few days left and will see a lot more participation.
Oh, I knew there was one other thing. Personally I fell like the "Advertising" part throws me off. Sounds more like an advertising firm in the more traditional sense. So think about downplaying it.
I hope this helps and I hope you get what you are looking for. Feel free to send any questions and comments my way.
L.