One thing you can do is add some text to the 'role-play preferences' page, many folks do that. For a One-on-One wanted, they do vary widely but some suggestions are:
Provide a little info on what sort of writing partner you are: do you post weekly? Daily? Do you write several paragraphs a reply? Do you like to discuss plots over PMs before starting the story?
Let folks know what sort of stories you're looking for, in general, and what sort of role you usually prefer to play. Do you only play women? Non-humans? Do you want to write romantic stories? Stories with a lot of smut? Or almost none at all? Do you like kinky stories or prefer more 'vanilla' ones?
This general information will help you find folks who are compatible with you in your style and general preferences.
Then, you can do a section that is more detailed about things you like to see in stories: kinks, character types (submissive men, furries, etc), and genres you like to write in (sci-fi, modern realistic, etc).
Then, if you have some specific story seeds, write a paragraph or two about them like:
I want a story set in the Walking Dead world, where my character is a zombie who remembers who they are. I want a writing partner who discovers this and the two of them begin a very odd relationship. I'll play a female character and I would like a male character as a partner.
I'd like romance, action and a dash of horror in the story. Maybe we'll both become zombies together!
Many people fancy up their thread with inspirational images, ones that relate to the plot seeds, or ones that simply fit the feel of your thread and preferences. I think though, it's most important to keep things clear and organized, in whatever fashion you chose (lists, neatly arranged paragraphs etc) and to put your best writing foot forward in your one-on-one add – after all, it's one way people are going to determine if you're a good writing fit for them. Using the formatting helps to add some structure to your thread so people can find what they're looking for easily.
And, as has been mentioned, the subject line is a great way to give folks a bird's eye view of what you want. Make it genuinely descriptive, not cute or catchy.
F looking for F and sci-fi action with lots of kinky smut!
Invader Zim Loves You! (this doesn't tell anyone what you're actually looking for)