While I am not personally strictly vegetarian, I do a lot of veggie cooking because we don't cook meat in our apartment (E. is veggie and the smell makes him sick).
Mal is right on with her beans ideas. I stock up on cans of beans to add to lots of things. I have recipes for vegetarian chili and a Mexican lasagna up on other threads in the cooking forum as well as a recipe for tortellini soup.
The biggest thing with cooking vegetarian is learning methods. Learn to make a marinara sauce. Learn to make a white sauce. Learn to adapt recipes with meat to vegetarian (my Mexican lasagna recipe is an adapted recipe courtesy of Rachael Ray). If you do eat eggs and cheese, figure out ways to use them. Omelettes are always delicious and filling.
Mal's wraps idea is one we do a lot around here. We buy a giant pack of tortillas and a bunch of fresh veggies. We chop up the veggies and put them in containers in the fridge. Those, along with a little bit of flavored cream cheese, make a great wrap. You can use everything from bell peppers to green onion to romaine hearts--I love to wrap a whole romaine heart with some other veggies. We also use hummus and tzatziki sauce on wraps when we are going Greek.
Stirfry is also a really good meal we do a lot. Frozen stirfry vegetables are awesome.
Rachael Ray has a lot of recipes that are easily adapted for vegetarians. The BBC's food site has a whole section on cooking vegetarian. I'd say the biggest pitfall is recipes that overcomplicate things. There's a lady out there named Crescent Dragonwagon (can we say hippie) who does this. I don't WANT to dirty 10 bowls when I cook, so always read the recipe thoroughly and make sure it's something simple enough.
Good luck
