I've definitely faced the same problem, and my solution is pretty much what Fat Dude was advocating. No matter how many sandwiches I bring, it doesn't fill me up (or excite me), and I end up starving by 2pm. I really need a "real meal" at lunch to last the rest of the day. So whatever I'm making for dinner the night before, I keep exactly half for lunch the next day. Pop it in the microwave, and my main course is taken care of.
Of course, for balance, I also bring (like someone else mentioned above) at least two fruits (usually apple and banana) plus a vegetable (usually a carrot, sometime half a sweet red or green pepper), plus a little thing of yoghurt. And if I have any dessert on hand, heck, I'll bring some of that too. I graze on these other things during the day -- some of it in the morning, some of it after lunch.
Things that work well for the dinner/lunch double are any type of pasta (just mix the noodles with the sauce *before* they cool down, then it won't get all stuck together, and will reheat nicely), any sort of stirfry or curry with rice, pizza (homemade stuff can be pretty healthy if you just buy the crust), and so on.
Of course, sometimes you'll be out for dinner, or having something (like tacos) that doesn't lend itself to leftovers for lunch. On those days, you can resort to sandwiches. Alternately, I actually buy frozen dinners (stuff like Uncle Ben's Rice Bowls, or even Lean Cuisine). You don't even need to keep them refrigerated or frozen until lunch -- they last fine. Obviously, I'm not a big fan of these, but if you combine it with the aforementioned fruit, veggies, and yoghurt, it's not a bad meal -- definitely better than fast food, and actually cheaper.
Okay, so occasionally you'll still have to go out for lunch. Even if you settle for fast food, still bring the fruit and veggies to snack on during the day -- that way you might opt for a regular burger rather than the triple-decker.
Last point (I promise): keeping all the fruit, vegetables, yoghurt, and other snacks (nuts and raisins are a good one) on hand does cost some money. But I guarantee it's worth it from a health/performance standpoint. And don't nickle and dime yourself by buying cheap crappy yoghurt and moldy apples: get the good stuff so you'll be appetized the eat right, and you'll end up saving money by not eating out as much.
Sorry for the long rant!