Sorting through the chaos...
The first thing you need to do is sorting. Take some large envelopes and label each one with the categories you will be using, we have success checklists here that can help you get ideas of what categories to use. Sort all your receipts into the categorized envelopes and then tally the amounts up, preferably with a printing calculator to save the printed calculations, and write the total on the envelope. Then it's a good policy to add the receipts up a second time and double check your number you wrote on the envelope.
Verifying Tax Forms
Next make sure you have received all the tax forms you should get- if you have worked multiple jobs did you get a w-2 or 1099 for each one? Did you move during the year and possibly not update your address with an employer or bank? Make sure you've opened all your mail and that you have kept an eye out for corrected forms. If you do get two forms from one company check to see if one of them is a "corrected" form, if not check to see if the account numbers are different. If in doubt call the company that issued you the tax form- whatever they sent you they also sent the IRS so you have to report those numbers exactly on your return and if the company screwed up (it happens a lot) then getting them to fix it now is a lot easier than getting it resolved once the IRS sends you an audit letter.
Compile Tax Questions
Make up a list of questions for your tax adviser or to research answers for if you are self preparing. Put together any articles you have printed of tax law changes you have questions about, and think about if anything has changed in the last year or will change this year that you need to talk about the tax implications of. Changing jobs, unemployment, self-employment, buying or selling a house, foreclosure, debt issues, investment decisions, all good things to ask about when working with your accountant. If you aren't taking advantage of asking questions to a professional you are missing out on half the benefit of hiring someone.