Siamese cats are fairly notorious for developing dental plaque that sometime leads to tooth and gum problems but which more often leads to absolutely horrendous breath. My former roommate's little Tonkinese, Simon, has breath that would spoil pork.
Feeding your cat dental crunchies can help, but my veterinarian said that a diet of plain dry food is best, as the special dental diet kibble can irritate some cats' stomachs and cause them to throw up more frequently.
If your Siamese is one of the very, very few that will allow such an indignity, you might try brushing his or her teeth. Your vet can provide you with a little toothbrush and samples of chicken-flavored kitty toothpaste.
A professor I know owns a full-blood Siamese named Blue. Blue also tends to have Death Breath. The prof kept losing his black nylon dress socks. One day he checked under his bed for a pair of lost shoes and discovered that Blue had hidden a cache of about a dozen of his socks there. The cat loved to chew on his balled-up socks. Concerned that the cat would tear loose a piece of fabric and choke on it, he took the socks away from the cat ... only to discover that the cat's breath turned to mustard gas over the following month. So now Blue, who won't tolerate toothbrushing, gets a sock every few weeks.
There is a strange behavioral trait I've observed amongst some Siamese cats: they hop.
The Siamese Hop is difficult to adequately describe. I first saw it in Simon. He crouches down on the floor as if about to jump on something, then alternately rakes his back claws through the carpet or across the floor repeatedly (he is particularly fond of hopping on surfaces that will create a lot of noise).
His rear end bounces up and down like a lowrider car with trick hydraulics. He looks like he's revving himself up for a race ... except that all he does is hop.He hops when he's excited or upset. If he wants in your lap and you brush him off, he'll run to the corner and hop, glaring at you sulkilly. If you drag a string along the floor, half the time he'll chase it and pounce ... but half the time he'll chase it and hop at it.
I think this trait would hurt Siamese cats' chances for survival in the wild. I can picture Simon stalking a bird, racing toward it ... only to stop and the last second to hop at his prey, thus allowing it to fly away.
Seeing Simon hop for the first time was a source of great amusement among visitors to our apartment. My roommate's mother nearly fell over laughing the first time she saw it. Part of the effect comes from the look of crazed intensity that comes over the little cat's face when he hops.
I at first thought that the hop was a peculiarity unique to Simon, but hy husband reports that he's several other Siamese do the same thing. He used to live with a cat named Tasha who also did the hop ... but only when she was about to hork up a hairball. It's a good enough early-warning system for hairball cleanup, I suppose.