Even when you're flat out with commercial work, as a creative it's inevitable that you'll think of ideas for personal projects along the way. It's important to fit these projects in when you can, because they're a valuable way to boost and diversify your portfolio.
Ben Cox, MD of Central Illustration Agency, recommends working on self-initiated projects that reflect your interests, because not only will this work express your personality, it will also enable you to feel more comfortable discussing your portfolio with potential clients and could help shape the kind of commissions you receive in the future.