"I couldn't do it" is something that veggies hear a lot, and if I get a chance to say that I used to think so too, the person saying it never seems very pleased to hear that! I think people use it as an excuse. They believe that being vegetarian is "good", but they think it's okay that they don't do it, because they just like meat too much. And I was once one of these people, so this is by no means a judgmental take on it. I only just realized what the psychology is, and it's based on a misconception the depth of whose falsehood has only recently become clear to me.
Omnivores, in most cases, are suffering from a failure of imagination. They're imagining their current diet with the meat removed, or replaced by tofu (which many people have never had cooked in a good way). They're thinking of a lot of plain, boring vegetables and salad, white starches, and boring cheese. Or they're thinking of hippie food stereotypes -- endless granola and sprouts. Or endless beans and rice. Heads up: this is not how vegetarians have to eat (although maybe some of them do). Even if you've seen a lot of vegetarian stuff and you think you're pretty familiar, there'll be gaps in your understanding.
I will freely admit that when I started my experiment with eliminating my meat consumption, I was probably thinking about one of these things. I was totally skeptical. Although I've never liked lamb, plain steak, or plain pork very much, I loved ground beef and beef in pieces (stir-fry, beef stew), chicken in all forms off the bone, turkey, ham, bacon (if lean), pastrami, pepperoni, etc. I didn't have any idea how I was going to go without those things. Luckily for me, I conceived what turned out to be a great idea: do it in stages, and learn as I went along. I didn't want to become one of those vegetarians who only eats cheese and junk food -- even in the infancy of the impulse, I actually did want to eat more vegetables, more variety.
Honestly, I think this is probably the best way to change a diet in any direction, unless you're under time pressure (eg you had a heart attack). I stopped eating red meat and pork first, followed later by poultry, and even later by fish, so I've only been a pure vegetarian for six months or so (though I never ate much fish anyway) even though I've not eaten beef in about four years. Even within that, I stopped some things faster than others. I ate lunch meat for longer than any other kind of meat, and tuna longer than any other type of fish. I stopped eating things in my own cooking long before giving them up when going out to eat -- for example, I've not cooked a fish or chicken of my own accord since 2002 or so, but I ate both in restaurants until 2004 or later. I've basically been learning vegetarian cooking since 2003, when I started cooking for myself in Scotland. And that's the key.
Not necessarily cooking, although I think that's hugely helpful and makes an enormous difference, but eating new things. I cooked or made one or more new recipes a week for all of the 2003-2004 school year. After that I recycled a few, but kept exploring. I continued my hunt for good cookbooks. I gradually experimented with veggie equivalents for things like lunch sandwiches. In the course of this, I also tried a lot of ingredients I wasn't terribly familiar with because I had disliked them as a child or one of my parents did, or my parents thought I did, or just didn't use them. Chickpeas, eggplant, lentils, split peas, leeks, fennel, barley, oats (which I never had other than in cookies or instant oatmeal until recently), rutabaga, turnip, parsnip...I don't remember them all, but I discovered lots of new foods. I learned to substitute things I like for things I don't, like zucchini for mushrooms. I learned the importance of herbs and spices. This has continued recently with my introduction to quinoa (a Peruvian grain, via Michael), udon noodles (Japanese, ditto), black lentils (via vegetarian haggis), and wheat berries (popular in Italy, via tha Barefoot Contessa (transitively via metacub, who introduced me to Ina's show)). I discovered that I love tofu, which is certainly an advantage for a vegetarian, though not required. I discovered that I don't like butter beans, pinto beans, or white beans, so my bean dislike still exists, it's just less wide and less extreme. :-) I've generally tried not to depend on things that are like meat but vegetarian, eg veggie burgers. I find them limiting to the kind of creativity that I've found to be essential in the transition away from meat. But I do like Quorn and sometimes use it in stir-fry. And veggie burgers (or plain beans and rice!) are fine for a quick standby.
Recently I've been applying this same method to attempting to become vegan. As before, I started out skeptical. I'm still checking out brands of soymilk; it's not my favorite. Rice milk is ok but not great for you. Tofutti "Better Than Cream Cheese" isn't, but it's definitely a fine substitute. But as before, the main key has been and is to explore new foods and new recipes, or adapt recipes that transfer easily. If all the things I eat and enjoy on a daily basis are rich and varied enough to satisfy me, and they're all vegan, then I become vegan "by default", though it can only happen through an application of effort in finding each individual new dish and ingredient. And only gradually. If I had to become vegan tomorrow, I'd succumb to the same failure of imagination that troubles most omnivores who think they "could never do that": I'd imagine my current diet with no eggs and milk, and fail at conceiving a true fully vegan diet (despite having eaten about 15% of my meals over the last few months with a vegan, and many of my own recipes being vegan anyway).
So, if you think it's a good idea to be vegetarian, or eat less meat, but you "could never do that" because [fill in your reason here], consider giving it a try. A small, gradual, experimental try. Buy a new cookbook, try a veggie sandwich at your lunch place, or buy a vegetable you've never cooked before and look up a recipe on the internet. (Use a food blog or good food site like the BBC, Food Network, or vegetarian magazines, if you can; general databases are of wildly varying quality.) Try adapting a favorite meat recipe -- I make a great veggie chili, and I just got a good recipe for lentil tacos down. In fact, why don't I list that one in case any of you are feeling adventurous today...
1/2 c each black (beluga) lentils and brown lentils
1 bell pepper (or 1/2 ea red and green, for color), chopped small
1 medium onion, chopped small
Oil for sauteing
Mexican spice mix to taste (~ 2 tbsp I think)
Water for cooking (2 c)
Taco topping material: chopped tomatoes, shredded cheddar, minced scallions, shredded lettuce or chopped cilantro, sour cream, whatever you like
Taco shells (or chips or tortillas (lard-free) if you prefer)
Cook lentils in 1 c water each, until tender, approx 40 minutes (black may take longer, but mine didn't). The black will be firmer and hold their shape more. Don't try to cook them to be the same as the brown. Drain the brown, drain and reserve the cooking liquid from the black.
During last cooking phase, chop the veg. Heat the oil over med heat (a few tablespoons...I don't so much measure stuff usually :) ) and add the onions. Saute 30 sec, add some spice mix, continue sauteing until beginning to become translucent. Add the peppers, saute until looking lightly cooked. Add the lentils and more spice mix, and a little of the reserved cooking water. Cook for 5-10 min, adding more cooking water as needed, and spice as desired. When the brown lentils are a pasty base for the mixture, you're done. Serve garnished with toppings.
This recipe is mostly vegan; to keep it all on the up and up, use soy sour cream (no cheddar, or use soy cheese, but I hear it's not good, so I'd recommend avoiding it unless you're hardcore) and make sure the spice mix and the shells or tortillas are vegan. Yes, some spice mixes aren't vegan, nor are many tortillas -- they can contain milk protein.
Addendum: In the comments, enshanam points out this lovely post by Barbara of Tigers and Strawberries. Some food for thought for those of you chowing down on grocery-store meat...