Siamese cats do not need to be groomed that often. If you want to groom them however do it sparingly. The most popular grooming area for Siamese cats is the ears. Usually grooming the ears includes trimming the hair on the inside of the ear. This causes the ears to appear much larger than they are. All you need to do when trimming the hair on the inside of the ear is get a pair of safety scissors and just cut the longest hairs. This just leaves shorter hairs which gives it a more natural look.
For the most part Siamese cats do not need a bath that often. They are usually not outdoor cats so their coat does not get dirty. If you do bathe it though, you should bathe it a the beginning of the week so the cat has the rest of the week to regain its natural oils in its coat. Also your Siamese should have proper exercise and a good diet to keep its coat nice and glossy.
Brushing is actually not good for Siamese. To much brushing can damage the color of the coat. The alternative to brushing the Siamese is to get your hands wet and rub them backwards in to the cats coat which takes out all the loose hair without damaging the coat.