As others have said, have all your papers in order but also with a summary spreadsheet to save accountants adding it up and charging you their hourly rate.
If you buy capital items, make sure you include information of what it is, date you bought it and for how much. Accountant will depreciate these so needs this information to do this.
Remember for personal returns on a cash basis, a common mistake people make, particularly with dividends, is giving us the dividend statements which relate to the period for 30th June, 20XX but are actually paid in the next financial year. This means then accountants spend time logging onto the share register to get your information. So it's the date you receive them, not which year they relate to. Same with bills for investment properties etc. It's the date you pay them, not the due date or date you receive them. These may sound like obvious things to most, but unfortunately they are small mistakes I encounter time and time again.
If you buy/sell any assets, include the date you bought them, cost, date you sold them and proceeds. SO many people forget to include the purchase details, particularly for shares (I'm sure people on this forum are smarter than your average Joe though) when they sell something. This means putting the job on hold, chasing up client, then picking it up again. More time wasted.
Wouldn't hurt to have a look at your tax return for the previous year to just refresh your memory and make sure you have included everything.