Here are some guidelines for keeping your site organized and easy to use.
As you build the site think about any parts that you may want to use in several places. In your layout especially you can keep your overall structure simple and easy to revisit by breaking the layout up with a few partials. Partials and pulled into a layout with the {% include %} element.
It's a good idea to keep each partial as a self contained unit that can be dropped in anywhere. It's a bad idea to split tag opening/closing between partials.
The regular layout for your site may well contain a mailing list sign up form. In fact, it should! If it doesnt', go add it. Now! OK, now in addition to your regular layout, you should have a version of it that does not contain any forms. This version will be used for error pages generated by bad input into one of your forms. You see, if your sign up form is in a sidebar, or footer on your site, if there's a problem with a visitors email address, they may not spot the form and the error in the normal location for the form.
So if there's a problem you want to direct them to a fairly basic page that reports that there was an error and that gives them a chance to correct their info. If that page also contains your sign up form in the layout, your vistor might be confused by the duplication. You can use the {% form_error_page page_handle%} element to specify which page to use as the error page. That page should {% include %} the same form partial that is used in your normal layout.
Alternatively you can use the {% if condition %} element to test the handle of the current page that's being displayed to decide whether or not to include the form.
It may also be useful to restrict the amount of navigation that you place on this error page in an effort to direct your user through the error. In the simple layout discussed above you can just leave out some of your menu include statements.