Hey there! There are a bunch of different ways to make great fajitas at home. Here are a couple of tips that will help you:
1)Most importantly to get the right taste, you need to use the proper cooking technique. To do the technique correctly in your home kitchen, prepare yourself for a smoky house and a slightly splattered stove-top. So have your vent flowing and/or your windows/doors open--and be ready to fan your smoke alarms with towels if they are sensitive. Also, use a heavy cast iron pan or skillet so you have the best heat retention possible.
Place the pan over high heat until it is very hot (usually when it starts to smoke, it is ready.) Now take only a few pieces of your chopped meat and veg and throw it into the pan. Toss the meat and vegetables around until cooked to your desired temperature (medium-rare, for instance.) Then remove those pieces from the pan and let rest, covered loosely with tin foil, in a separate dish. Be sure to get as much of the food out of the pan as possible to prevent them from becoming burnt and making their way to your dining table. While this first batch of meat is resting, repeat the process with another small handful of meat/vegetables.
By cooking your fajitas in these very small batches in a very hot pan, you allow any moisture from the meat/veg to cook off quickly. If you add too much food at one time, the meat won't brown well and instead cook as if you steamed the meat (light brown/gray in color with no dark browning.) Don't be misled by the fact that restaurants serve you from a small skillet. That is for presentation only. Most likely, the meat was cooked on a very hot griddle (meat spread out so moisture is not an issue), or over an indoor grill; then, to serve, the meat is chopped and placed onto the small skillet that has been pre-heating under the broiler to give the dish that sizzle effect as it comes to the table. In this case, you are simply making the "sizzle" during the cooking process.
2) Using the proper meat is also essential. Refer to my article on Carne Asada for a brief overview on locating flap meat/skirt steak: http://www.cheftalk.com/wiki/mexican-food-recipes-carne-asada Also, this article describes a basic dry rub marinade that adds a nice flavor without overwhelming the beefiness. But to give you a baseline flavor to start with, why not start with salt and pepper only? Then you can add more seasonings as you refine your fajitas. To adjust the recipe in the article for fajitas, simply cut the meat into 1 to 1 1/2" strips prior to cooking. If you are fortunate enough to have a quality Mexican grocery store nearby, they will normally have meat that is pre-marinaded and pre-chopped for you. So just add your desired vegetables at home and you are ready to cook!
I prefer to not add citrus juices to my fajitas until after the meat is cooked. I use it more like a seasoning right before eating by adding just a few squeezes across my meat. The citrus cooking at those temperatures give the meat a slightly odd flavor IMO, probably due to concentrating the acid. But if you have a great recipe that calls for juices prior to cooking, go for it! There are bunches of ways to make great fajitas, and finding the one you like best is half the fun!