Well, depends. If you have LOTS of sprites that change, you could just keep the background in a buffer and blit (memcpy, possibly line-by-line) it all to the main buffer instead of clearing. If you really only want to update the overdrawn parts, you will need to keep up some form of "dirty rect" scheme. Dirty rects are rectangular parts of a buffer/sprite which are known to need updating. This means you save the coordinates of every sprite you blit and blit that part of the background image to the back buffer. If you don''t know what I mean, either ask or (better) do a google search on dirty rect, I''m sure there is some info to be found. A dirty rect scheme is no advantage if you''re drawing a LOT, especially with overdraw, as you''ll spend more time copying the same parts of the screen back and forth than actually drawing the scene, so it''s often better to just blit the whole background.
A variation of this is to have a buffer for various parts of the scene and doing the same with that, like, you have a tiled landscape and don''t want to update it every frame because it always stays the same - just have a seperate buffer in memory.

- JQ
Full Speed Games. Period.