You need a waiver and release of liability form. Most of the ones you'll find online will be for accidents. You need one specifically for intellecutal property - graphics.
But, I'm wondering - aren't you designing this work yourself? Are these your ideas or are you getting art work from your clients? Can you clarify a bit about your process?
If you're creating something from scratch, as a client, I would be really skeptical about signing a release saying that I take on full responsibility for something you created.
For copyright infringement, you're only in trouble if you copied someones work. As long as you can prove independent creation, you'll be fine. Trademark infringement is determined a bit differently but it is highly unlikely that you would recreate someone's trademark and not know it... otherwise, it's not much of a trademark is it?
Something other's didn't mention - you should get insurance to cover you for any such lawsuits. Keep in mind that general business insurance will NOT cover IP lawsuits. In fact, IP is usually expressly excluded. So, make sure you read your policy.