How do I create optional sidebar to posts

I am creating an online portfolio by customizing a WordPress theme.

So far, I seem to have got everything as I want it – but now I come against the problem of either adding sidebar a sidebar to ALL single post pages, or none. And, if added, that sidebar would have the same content for every post.

What I want is the option to add a sidebar. And I would want to add specific content on each page.

So far, the method that has worked best is to amend the single stylesheet to make each post the width of the theme – then add a table to the html part of each post which allows for a post area and a sidebar area. But this seems a bit – umm, ‘clunky’.

The other method I thought of was to do the same, except add div tags to the html of each post page – that’s proving tough because of the hierarchical nature of css, I am not getting the desired effect.

Does anyone have any suggestions as to how I can achieve this? Once the site is done it will not be added to very frequently, so I don’t need to worry about adding bits of code each time I post – I just want it to look right.

Thanks.