Most days I have no problem packing lunch, but seriously, other days, I'm dragging--hard. All I can think about in the morning is coffee. Sometimes I'm running so late, lunch is completely forgotten. This happens to everyone, so there's no need to stress when you forget your lunch at home. Forgetting a homemade lunch won't ruin your healthy eating habits!
What to Do
Actually, first, let's talk about what not to do. Do NOT settle for the most convenient and most cheap menu item. This should basically be screaming, "Hi! I'm unhealthy!"
Instead, try finding a place where you can "make your own" meal. This way you know exactly what ingredients are in it, it's fresher, and it can more accurately match your current eating style. Look up places to get food around you, scope out the menu, and try to pick a spot that has food that will keep you full till dinner.
How to Not Forget Lunch Again
Sticky note on your fridge: leave yourself a note if packing a lunch typically slips your mind. Seeing that note every day will help remind you to grab your lunch before you head out the door. Hopefully, if you continually remind yourself, it'll become a habit.
Pack your lunch the night before: save yourself some time (assuming you run late in the mornings, like me), and pack your food into baggies and containers the night before. This way, the only thing you have to worry about is putting all your food into your lunch bag.
Hang your lunch bag on the door: if your lunch bag is on the doorknob, there's no way you won't see it when you go to leave for work. Hopefully, you've packed your lunch the night before and can quickly put everything in your lunch bag.
Now, what do you do if you still manage to forget your lunch, and don't want to spend money? Always have some snacks stashed at the office. At my desk, I have a stash of Nutri-grains, peanut butter crackers, and nuts. These normally take the edge off and can hold off the hunger till I go home for dinner.