Rails validation scheme: why not validate params directly.

Hi. There are many ways of doing validation but I'm trying to think of
the cleanest way without making the action code unclean. Would it be
advisable to do validation on the incoming params hash rather than
waiting to setup an object you are about to save and calling ojb.valid?
before saving? Or even going as far as trying to save the object but
failing from the model's validation scheme?

Why validate params when your model knows better how to validate already?

The place for most validation is within the model. There is some validation which is better done at the controller level, so this isn't an absolute rule. If validation fails, the model won't be saved, so an additional check on valid? isn't normally required.

Handling errors - How do people display errors cleanly? Do they use
flash[:notice] in a try/catch block in the action to catch the
validation exception?

No, just conditionally display the flash within your normal view template, and set it from a check on your save result.

Any help would be appreciated on designing this. I don't want to mess
up the actions with all the possible erorr messages etc.. This jsut
makes the code very unreadable.

You could get very tricky with checks on all sorts of things, but it's usually better to go with the flow and just let Rails handle it. If you have followed the examples in the AWDwR book, you will have seen that this needn't result in overcomplex code.