If you have something you can trademark like a slogan, logo or design name, you can protect these by filing a trademark application.
In the UK or EU you will immediately get an application number. It may take months to actually come through, but at least you have a filing date that is earlier than anyone else.
This gives you the legal power to sue. It means you can warn people that as soon as the trademark is granted, you will be able to initiate legal action against anyone using your ideas.
Ask clients to sign a confidentiality agreement that says the contents of your presentation are confidential, and that all IP rights are owned by your agency.
Make sure that this contract is properly worded, and covers things like how for long the confidentiality is required.
Make sure you can show an ideas trail. Create a document containing the idea, date and logo. Send it to yourself by fax or by registered post so that you have official proof that you created the idea earlier than the presentation.
If you put a copyright logo with the date on your materials, this will also protect your content as long as your work is not copied originally from someone else. This is automatic and does not require registration.