Approach #1:

Use the Cartoon w/Lines document display mode (CTRL-7).

Advantages: cel-style shading with lines
Disadvantages: no textures, transparencies, bump maps, or shadows

Note: From the Display menu, you can set the display mode for individual elements (figures, body parts, etc.) of your scene, allowing you to mix-and-match modes simultaneously. So you could have texture-shaded eyes on a cartoon-shaded figure. Also, Cartoon w/Lines looks better after you antialias the document window.

Approach #2:

Play around with the Sketch Designer. I don't think you can truly achieve a smooth cel-like cartoon render with the thing, but it's great for creating renders with a rough, penciled or painted appearance.

Approach #3 (Poser 5 only):

Learn how to utilize the Material Room's shader nodes, especially the Toon and Edge Blend shaders.

Advantages: Can be rendered normally, with textures, transparencies, bump maps, and shadows.

Disadvantages: The lines aren't quite as bold as those in Cartoon w/Lines mode.

Note: Disable the regular Diffuse_Color channel by setting its value to zero, and plug the shaders into the AlternateDiffuse channel instead.