Generally, anything you can argue is a necessary part of your business will be deductible as expenses. This would include specialist magazine subscriptions, memberships, conventions, and so on, because they're part of your professional education and development.
As a rough example from my own freelance editorial work, I used to be a member of the Society of Editors and Proofreaders, and deducted my membership fees as a business expense. If I'd travelled to London for one of their courses, all reasonable travel expenses would be deductible, as well as the cost of the course itself
Clothes are a bit more problematical, as was
reported in the Telegraph today. The newsreader Sian Williams tried to claim clothes (and hairdressing) as expenses. It was decided that she would have to wear clothes anyway, and the things she wore to work were ordinary clothing, not bought "wholly, exclusively, and necessarily" for the job.
So a nice suit wouldn't fly as a business expense, but full-blown theatrical wizard robes would.