My go-to is Subway. I can get a veggie salad or a veggie sub made the way I want it.

I usually look for their menu online and then try to call ahead to give them a heads up.

Sometimes the staff gets into it and I get amazing food and sometimes it is a disappointment.

Salads are always an option as are sides. Plain pasta with olive oil and tomatoes can work.

Hardest for me is breakfast - I know oatmeal is an option but I can get tired of it. Recently i ordered eggs benedict and asked then to hold on the eggs, ham, hollandaise sauce and any butter. Instead they sauteed spinach and tomatoes and put them on a toasted english muffin - pretty good.

checking an online menu ahead of time is definitely useful! also, happycow.net gives you stores and restaurants near you that are vegan, veg-friendly, etc. and you'd be surprised at what you'd find.

usually restaurants and food service places are pretty flexible, but you do have to be careful for things that are not blatently said on the menu. for example, when i first went vegan i ordered a pesto pasta at a restaurant because when i make pesto at home it's vegan, so i didn't know that most people include cheese in the pesto sauce. not a big deal, but it's best to always ask the server if they have vegan options.
sometimes they can adjust things and it won't be included on the menu.

i always try to carry a nutritional bar or some nuts or dates, or other portable snacks in my bag or when i'm going anywhere. the #1 thing about staying vegan is to have food when you're hungry. even if i'm going to a friend's house i bring little snacks, because most of my friends aren't vegan and i never know what they have.

if dining out with friends, don't be afraid to call ahead and ask to speak to the chef and see what kind of alternative options they could make for you. chefs usually are really nice about this type of thing, and are often relieved as their job can get repetitive cooking the same menu dishes everyday.
one thing i've learned about social dynamics and veganism is that my non-veg friends are surprisingly open-minded to trying vegan food. we go to vegan restaurants and they LOVE the food. we also have potlucks and i'm constantly impressed at their efforts to make everything vegan so that i can eat it. it's a great way to have your friends be more open-minded and knowledgeable about veganism, and also be able to think about transitioning in the future.

good luck!

I too am a big fan of sides a bowl of green veggies with some toasted almonds and some Rosemary potatoes and I am a happy girl!