First, be sure to hit your Daily Dozen regularly. A properly executed front squat requires that the whole body works systemically to perform the exercise. The daily dozen will help you gain more full body mobility. You may also use straps around the bar to help with your rack position until you acquire the necessary flexibility to hold the bar properly. If you perform the wrist series in the following video regularly, it will dramatically increase your range of motion.
Now that you have some drills for the wrist and forearms, you need to get the elbows, triceps and shoulders moving right. To do that, it helps to foam roll the triceps, lats and ribs while stretching the triceps and obliques, as they all tie in together and will move better with some manipulation. This is outlined well in the following front squat tutorial video.
If you're only doing front squats to big bigger legs, it would be fine to hold the bar with crossed arms and not worry about flexibility. But if you're an athlete and use the front squat as an assistance exercise to the clean as well, I recommend you avoid the body builder crossed arms hold, and learn how to rack that bad boy on your shoulders properly. A proper rack will also force full protraction of the shoulder blades and proper extension of the T spine, which does not occur when crossing your arms. Also notice in the photo, Dave Draper is squatting in loafers and putting a 2x4 under his heels. Loafers are a flat shoe which keep you conected to the ground without a lot of cushion like todays running shoes, which makes them a decent squatting shoe. The elevation of the heel with the 2x4 would be necessary for many athletes who lack adequate ankle flexibility. Over time you could replace the 2x4 with a 1x4, then nothing at all. Or you could use Olympic weightlifting shoes that have a built in elevated heel.
Benefits of front squatting
How to front squat