1. Choose the right skewers
You have two choices and both have their strengths and weaknesses. Metal skewers can withstand the heat of the grill and can be washed and reused, but they get very hot. If you're planning to serve kebabs to guests directly from the grill, wood skewers are a good alternative. Wood skewers won't burn your fingers, but they must be thrown away after one use and they char easily. Many cookbooks recommend soaking wood skewers in advance to keep them from scorching, but I've found that doesn't work very well: The heat of the grill usually dries out the wood quickly and the skewers burn anyway.
2. Use two skewers per kebab
Threading ingredients on two parallel skewers can be tricky to master, but this method ensures that the pieces lie flat and don't spin when you turn the kebabs.
3. Cut ingredients into equal-size pieces
On the same skewer, put similar-size pieces of vegetables, meat and fish that cook at the same rate. Nothing's worse than a kebab with dried-out shrimp and undercooked onions. If you're using shellfish, like shrimp and scallops, cut the other ingredients to match their size, and be sure to skewer them with only quick-cooking vegetables.