The most helpful tips will vary depending on how/why you slip up.
If it's because you find yourself out and hungry with nothing but nonvegan food to eat and about to faint from famishment, try to keep some snacks in your bag at all times - nuts or fruits (fresh or dried) or a healthy snack bar - things that are high-energy and that can tide you over until you *do* have access to good, hearty vegan food.
If it's a slip-up because you crave or miss certain tastes, you could try a more gradual weaning off these foods. Many people find it helpful to remind themselves of why exactly they opted to go vegan - keep whatever your ethics, interests, or inspirations at the front of your mind.
Educate yourself about your vegan alternatives and keep them on hand. Look at the dairy and egg substitition threads on here. I think there's one particularly about vegan cheeses. Search elsewhere online for information. Look at vegan recipes, many excellent ones are posted on www.vegweb.com
When I was transitioning from vegetarian to vegan, moving away from baked goods made with dairy and eggs was hardest step - but I overcame that when I learned what substitutions to make in my own baking. Of course, I still can't have scones and cookies from mainstream bakeries and coffee shops, but I console myself with the fact that they're laden with sugar and hydrogenated fats anyway, and am healthier overall for *not* having such easy access to sweets.
You also asked for suggestions for good vegan packed lunches. Try making wraps - a good whole wheat tortilla filled with any combination of a protein (tofu, seitan, tempeh, chickpeas, lentils, black beans, a veggie burger...) along with rice or noodles, and raw or cooked veggies of your choice, with a tasty sauce. Try making hummus - incredibly easy, and very tasty - and pack pita bread and veggie sticks along with it. Make sandwiches with a bread you like, tofu, roasted peppers (or roasted pepper spread). Make a pasta salad with whatever protein you want, and veggies.
All these suggestions hinge on some degree or other of cooking, and that's my overall suggestion. Of course, not every vegan loves cooking, or does a lot of it. I do, and I've found that being able to provide tasty food for myself saves me a lot of time, makes sure I can get balanced meals, and is a great way to shut up people who down me for being vegan. There's nothing like being able to shove a cookie or well-seasoned serving of manicotti into someone's mouth when they're taunting your lifestyle - especially when their only response afterwards is 'wow....is that really vegan?'