Pack more calorie dense food into your diet. Trail mix, granola, protein bars ( balance, cliff bars, zone perfect), nut butters ( a bagel with two tablespoons of nut butter on each side, easily four hundred calories) :)
I started meal planning for the day so that I wouldn't find myself in that situation, as I experienced the same in the beginning. It's a little harder when you are out and about though. The others posted great ideas. Thanks!