How do I deduct expenses that are part business and part personal, like my cell phone?

You deduct only the business-use portion of a mixed-use expense, based on a reasonable, documented allocation. For a cell phone, estimate the percentage of use that is genuinely for business, say 60%, and deduct that share of the bill. The same logic applies to a home internet connection, a car, or a computer used for both. The IRS expects a defensible method, not a guess, so keep notes or a log supporting your percentage. Fully personal use is never deductible, and claiming 100% on something obviously used for personal life invites scrutiny. When an asset is used more than half for business, you may also depreciate that share. Consistency year to year strengthens your position if questioned.