Mix a raw egg with some parmesan, a little soy sauce, sake, mirin, and a fair bit of Ikura (it's quite a subtle taste). A little wasabi is nice too. I just toss the hot spaghetti in the mixture. I am not scared of raw eggs (he said boldly), but there are concerns about salmonella (sp?) so you can return the pot to the heat for a few seconds if you want (I don't). Try using some buttermilk as an addition: it is light, but tastes nice and rich. I often sprinkle crispy nori (seaweed) flakes over this pasta just before serving. Tobiko (Flying-Fish roe) is good this way too.