It sounds so simple, but it's still overlooked by most people. Whether you eat three, four, five, or six times a day, include a source of protein with each meal. There are two main reasons:
What fills you up more, a sandwich or a steak? Research seems to indicate a strong protein effect on the hormone peptide YY, which is released in the ileum and colon in response to feeding and is involved in inducing satiation.
That's partly why it's so easy to eat endless bowls of cereal, but after 200g of chicken you'll likely have had enough.
The caloric cost of digesting and processing protein is much higher compared to fats and carbs. When you consume protein, 20-30% of the calories consumed are expended during processing. There's a high cost associated with unfolding proteins and digesting them, and it outshines carbs (5-6%) and fats (2-3%).
If you want to stay lean and in shape year around, start every meal with protein and you'll feel more satiated, burn more calories, and eat less overall.