Well it's essentially a pizza that is only dressed on half. Traditional styles are usually saucy with ricotta, perhaps some meat or peppers (nothing too fancy). But basically you can put whatever you want in it.
Some of my tricks are:
1) If going for a clamshell, keep a good 1-2 inch edge clean from the sauce. Otherwise it won't seal good.
2) I moisten my edges with a bit of water before sealing the top on.
3) You have to prick some vent holes or slash the top, otherwise it will blow out the side.
4) Par bake or cook any raw meats thoroughly before using them inside. Especially if you stuff it really full.
Good luck and be creative. Basically once you have the dough technique down in making the pocket and cooking it, the real art is in the flavors of what you put in there. Simple fillings with ricotta, mozz, garlic and fresh basil are nice. Others load them up with tons of different things. If you stuff it full, it's going to take longer to cook and you need to be careful it cooks before it burns. This is especially the case for #4 above.