Prepare the Shrimp:
In a medium bowl, combine the shrimp, baking soda, salt and sugar. Toss to coat. Thread the shrimp onto sets of 2 skewers, rotating heads to tails and threading the shrimp so they are snuggly pushed together. Each set of 2 skewers should fit about 6 shrimp. (Check out this link
) Transfer the shrimp skewers on top of a baking dish so they are suspended above the bottom of the dish. Place, uncovered, in the refrigerator and allow the shrimp to rest for 45 minutes to 1 hour. Meanwhile, in a small bowl, combine 1 ½ tablespoons of oil with garlic, cumin and chili powder. In a separate bowl combine remaining 1 ½ tablespoons of oil with the lemon juice. Set both aside.