I have this problem of taking portraits under yellow light. athe pic looks dull n shows a yellow haze as a result of the yellow light. They r not crisp n clear.

Also, i don't advocate the use of Flash for portraits as it steals the natural colour.

I have also tried using Filters with my Canon 400D but to lil effect.

Kindly advice me on this and oblige.

Set a custom white balance using a photographic gray or white card. You'll find how to set a custom white balance in your camera's manual.

You can use almost anything white in a pinch and get it close. For example, I sometimes use some white coffee filters stacked together as a target.

Using a custom white balance allows the camera to compensate for the temperature of the lighting.

As Jim noted, custom white balance is a good way to go.

If it is available, shooting with RAW is also a good idea. That allows you to easily adjust the white balance after the fact.
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But why are you shooting with a yellow light? Do you mean tungsten? If tungsten, likely your camera has a white balance setting: RTFM - Read The Fine Manual.

Post an example. That could be shake, bad focus, peanut butter on the lens, ...

Flash lighting can also be effective with the use of reflectors or diffusors or just simply bouncing the light.