She's more then half bald.

My 2 year old siamese, Amethyst, is darned-near more then half bald already from itching and biting her hair out. I've tried different foods, different litters, different detergents at home to no avail. The vet says it's allergies and the monthly shot she's been getting to help control the itching doesn't work anymore. I'm not sure which direction to turn. This has been going on since last summer. Please help! 

She may have an allergy to her food. I would check out the nutrition section and the raw feeding boards. A lot of people have found switching to a homemade or commercial raw diet helps with certain allergies and diseases like IBD. You could also try switching to a limited ingredient commercial canned  food. It is worth a shot before you try more medication, which may have negative side effects. I would also take a look at catinfo.org it is written by a vet and has a lot of information on cat health and nutrition and she also has a recipe for a complete homemade diet.

Another thing you may want to look into is fish oil. My 16 year old dog had a thinning coat and the vet thought she may have cushing disease ($800 I found out she didn't have it). I put her on fish oil and her coat started to grow back in!

She may also have a flea allergy. You may not see fleas but she only needs one to bite her to have the reaction. Also, fleas use the cat's digestive system in the life cycle, so you may see their larvae (little white worms that look like rice grains) near the cat's anus. Awful little pests.