However, registration is the best way to ensure protection of your character. It is up to you to judge how to get the best return for your hard work. In general terms, the key to maximising returns is to retain copyright and grant limited licences which reserve other rights for other deals. However, types of agreements differ and you should research the terms and conditions it might be possible to obtain from a particular licensee. You should also ensure the agreement is clear.
Many companies are loath to use written licences.
However, it is in your interests to confirm the arrangement in writing, particularly as the licensee will tend to interpret ambiguities or omissions in its favour. Arts Law's sample Image Reproduction Licence sets out terms including approval of sample images, the extent of the licence and the right to attribution. Your cartoon is appearing on thousands of mugs and t-shirts with no slice of the profits going back to you. Or, increase your upfront fee. Your cartoon is tied up indefinitely while a magazine exercises 'first rights'.