How best to "structure" freelance income?

I work full time, earning a salary and paying PAYE tax. Through the year I have some projects that enable me to generate some extra income. However, if I just do this as an individual, I incur additional income tax at my marginal rate. Is there a way to structure "freelance" work and projects (in something like a company?) so that I can pay less income tax whilst still abiding the law?

How much extra do you earn and what is the total of your salary?

For arguments sake, lets say that the annual salary is R300k, and the extra amount earned from freelance/contract work is R25k annually.

Given the amount of R25k extra there is no possibly structuring to be done. You will be able to deduct any expenses incurred in earning that income so it will reduce the effect of the income earned.