Making Packets: To prepare packets for the grill, take a sheet of heavy aluminum foil large enough to fold over the food you want to put inside. Place food in the center and fold up the ends. Make a double fold where the ends meet. Then fold up the sides to form an airtight packet. There should be room inside for the juices and heat to circulate. The idea here is that the food inside the packet will cook in it's own juices.
Packet Ingredients: Because of the way packets cook you need to make sure that you have enough liquid inside to create the steam that will cook the food. Water, wine, fruit juices, vinegars all work to create this steam. By choosing something with flavor you can enhance the taste of the foods. If you add dry spices to the packet, make sure you compensate with more liquid to keep the packet from drying out. When the packet has been cooked the steam inside can be very hot, so make sure you carefully open one end or puncture the packet with a fork before you take out the contents. Remember that steam can be considerably hotter than boiling water so it won't take much to burn you.
Grilling Packets: On the grill food inside packets can burn. The direct heat of a burner is more than enough to burn foods quickly and since you can't see inside while the food is cooking you can to be very careful. Packets need to be turned regularly and watched constantly. If you start to see charring on the outside of the packet then you probably have burning on the inside. Keep packets away from a high direct heat. Upper racks and cooler areas of the grill are best. You can also take some precautions in the way you assemble the packets. Try layering sliced onions on below and above foods so that the onions can absorb some of the extra heat and prevent the food you want to eat from burning.
Packets are great for more than just side dishes. Whole meals can by put together to allow easy preparation and to blend flavors. Try packing fish fillets, onions, lemon slices, and vegetables together with herbs and spices to make an individual meal for all your guests. You can even serve up the packets right on a plate for people to open and enjoy. Because of the sealed environment you don't have to worry as much about foods drying out, but be careful not to puncture the packets while they are cooking.