Or at least don't mail it until you make sure you have the right postage.  On Monday, May 12, the postage rate for a first-class letter is going up by one cent, from $0.41 to $0.42. 

If you remember, the most recent change was last May when the rate went up two cents.  It's no coincidence that the rates are bumping up each May.  Legislation was passed in 2006, to implement regular and predictable price changes in postal rates, as well as revamping some of the management of the USPS. (Docket No. MC2007-1, Order No.43 — Order Establishing Ratemaking Regulations For Market Dominant And Competitive Products: Download FinalRulesWeb.pdf)

I linked to the full document for the legislation above, but for an easier-to-digest version see the comments from the Postmaster General: Download GuidingPrinciplesFinal.pdf

As far as the rate changes go, there are more changes than just the first-class postage, so be sure to review the rate schedule if you mail anything more involved than your typical letter.