Barcodes are the mark of the beast. Have a barcode made that relates to the ISBN of your (comic) book. Normally you need to pay about twenty pounds per barcode, unless you have a friend who can do it for free.
Barcodes aren't essential for all potential retailers of your (comic) book, but, if you plan to sell them through big chains like Waterstone's or Borders, it will help your case. Amazon.co.uk won't sell anything without a barcode. Make sure the barcode is printed at 1200dpi.
Now publish your (comic) book. Make sure you think your (comic) book is good, and enjoy that feeling while it lasts. Because no doubt, in about a weeks time you'll think it's a pile of shit. And in about a month, you'll know it is.
Next you need to supply Whitaker and Bookdata with relevant information (title, author, isbn, content etc) about the (comic) book. Get a form from them or do it on the net. It takes months for them to process this information - so do it early.
Once processed, people will be able to order your book from bookshops around the country via a system called Whitaker Teleordering. Once again, many big chains won't consider selling a (comic) book unless it's available through the Teleordering System. Amazon.co.uk also requires this.
Once published, send a copy of your (comic) book to: The Legal Deposit Office at the British Library, Boston Spa, Wetherby, West Yorkshire, LS23 7BY with a letter saying you'd like to deposit it for copyright purposes. Give your address. You'll get a dated receipt from them in a few weeks. This ensures, in a pretty watertight way, copyright of your work.
Now, for posterity, (and also by law,) send five copies of your (comic) book to the: Agent for the Copyright Libraries. 100 Euston St, London, NW1 2HQ.
These will be forwarded on to the Universities of Oxford & Cambridge, The National Library of Wales, The National Library of Scotland & The Library of Trinity College, Dublin.
Now trudge around shops asking if they want to take your book on a sale or return basis.
Most importantly, learn to convert rejection into energy.