I've just completed my first tax return - did it myself online. Assuming I did it right - I'm guessing that my circumstances are pretty simple as I thought the process was all quite straightforward.
I enjoyed it actually - it's satisfying getting everything to reconcile.
My online invoicing system www.icq32.co.uk really helped me. It stores all my invoices, cash received and expenses - so I added a new report where I simply pick the tax year from a drop down and it displays all cash received and expenses paid in the selected tax year. I also added a button which selects all the report data for copy/paste into Excel.
It saved me so much time using an invoicing system like this rather than Excel spreadsheets.