Since I just got done sending in my taxes for this year, here are the categories of deductions my (very conservative) CPA is happy to include on my Schedule C:
Postage/FedEx/UPS
Printing Expenses (for pre-ARC manuscripts to reviewers)
Book copies (that I have to buy myself when the free books from the publisher are gone)
Misc./Gifts (for agent/editor holiday tokens, gift baskets, donations to causes)
Website maintenance/design
Print ads
Promo goodies
Office supplies/software/equipment
Dues & Subscriptions (trade magazines, organization memberships)
Professional Fees (Agent, entertainment attorney, accountant)
Meals & Entertainment (conference dinners, lunch meetings with other writers, etc.)
Book Conference Fees (just the dues. Other stuff goes other places)
Hotel/Valet (for conferences or book signing tours)
Travel (for . . . well everything. I think it's up to 42 cents a mile for car travel.)
Hope that helps organize your thoughts on what sort of receipts to keep track of.