XSI is great tool.
It has great animation tools for Toon animation, and also great shaders. It's easy to get a Toon look just in minutes. Even though it's that easy you do need to keep in mind a few rules. Never do your image character too dark. Black is not a color for you. Even the dark blues of night would look better, believe me. The colors need to be bright, but if your characters are too colored, use simplier enviroment for them. Find colors what will make the character stand out and visible. It's wrong to think that Toons must be colorful. Make it simple and attractive to the eyes.
Let's see step by step how I gave toon shader to this Robot.
First you need is apply the shader to the mesh. I used Lambert Shader. I love Lambert Shader, and in fact I don't need specularity now, because I will apply this with Toon Paint and Host Shader. The reason why I use Lambert Shader with Toon Shader, and not just Toon Shader itself, is that I can modify Color, Transparency, Reflection and Incandescene there. Also Incandense is very important for keeping the material in dark scense lighter. You can do this with ambient lighting, but I prefer this way.
So this is how the Robot look with Lambert Shader. Not Toony I know, but it's a simple thing how we can achieve Toon Look in XSI
I open Render Tree. You can see the render tree of current shader. If not select the shader under XSI explorer, and refresh Render Tree window.
Now add Toon Paint Shader from the Nodes menu, and connect Lambert shader to Toon Paint surface. Then to get it work, you need to connect the Paint shader to Material's Surface.
After that the shader should look like on the Robot like that.
Not that bad, but a bit flat at the moment, and some parts have bright specularity.
So open Toon Paint Shader and adjust a few things
...and enable and adjust RimLights. It gives you the nice outline effect around the shape.
So the last thing you need now to do is add some color and try what looks best. Add colors to Highlights, RimLights. Adjust the alpha and softness.
Also you can play with compositing. If you are familiar with Photoshop you understand well what all these methods mean.
There are milions of different results you can achieve with this. Play, explore all parameters, enjoy and have a fun with XSI)
One more thing at the end. The hard Highlights and Rimlights are best for Metal and that kind of materials. Use more softness for plastic and so on.