When you are just starting out as an illustrator and still not fluent in negotiating contracts, it may seem reasonable to simply deliver the work and get paid, end of story. This may be the case for "work for hire" type of jobs where the illustrator gives away their full rights of the artwork to their client in which the client is then free to use the artwork in anyway they wish without restrictions. This of course is a viable and legitimate type of commission but it is worthwhile to consider instead to work with a license.
License: Grants client usage rights specifying: nature of use, region of distribution, distributor and duration.
Clients unfamiliar with working with illustrators can get alarmed if they don't have the full copyright of the artwork but in actuality their concern is usually in regards to usage rights which is fully covered under the license. In the case of branded projects or projects focused heavily on character design, you can consider granting an exclusive license which reassures the client that the illustrator will not reuse or adapt the artworks created for them for other purposes.
In addition, by only granting specific usage rights in the license, the illustrator is able to leverage additional compensation for further usage rights. Know your value. You are a contributing factor to the client's potential profits.
When the illustrator keeps the copyright of any work they produce they are able to maintain the artistic integrity of the artwork to prevent unauthorized usage or alterations that could negatively impact their reputation. You are also giving yourself more opportunities for further work. The client will come back to you instead of handing it off to their in-house design team to repurpose your art for the next project. This is not only a means of protection but a measure of delivering quality work when the illustrator is fully involved and can let their expertise shine in the creative process. Try to explain this concept to reluctant clients through this constructive perspective that you want to be integral to the creative process together with them instead of emphasizing copyright as a defensive term against infringement. Put yourself in their shoes and think what kind of concerns they might have and always try to approach it from a collaborative stand point.
Whatever way of collaboration you choose to do: work for hire or licensing, understand the risks and advantages involved to make an informed decision. There is no hard rule to negotiation and sometimes the personal and emotional side of things will take the lead. Know that you will improve from these collective experiences and continue to do better.
Finally, I highly recommend joining the AOI (Association of Illustrators) for further resources and direct help on contract negotiation. Their 1 to 1 member support is amazing and helped me so much to confidently take on larger jobs. I believe strongly that a working illustrator cannot simply operate on art alone, the business side is essential to ensuring your continued growth. Professionalism and work ethics can take you far. Remember that even a single illustrator's efforts contributes to advocating for a fair creative industry!
Thanks so much for reading. I really hope this can be helpful in some way. All the best to your creative journey. Come say hi over on instagram: @natalie.illustration