Making seamless textures maps is purely a technical thing. Making good shaders by combining different images and procedural settings requires an artistic eye.

Technical: Just make sure that the T-Maps you use are large enough for your final output size. Do some tests.

Artistic: Don't make the shader from a single texture. Make a different image for the dirt splatters and put that in the highlight channel. Make a map for the reflections. You know what I mean? Real surfaces are made up of many substances, each having different optical properties. Shaders look best when data is different in all the channels so as to simulate the different amounts of highlight, bump, reflection and so on. Study surfaces in the real world. Try to figure out what the properties are that make one surface look different from another.

If you want a decent reference, check out this book: Digital Texturing & Painting.

It's pretty decent, particularly if you're new to making 3D shaders.

If you need more specific info, shoot me an e-mail, I'd be happy to gab your ear off about this sort of stuff. -pix