slight... confusion?

I see obituaries, or just remembrances, posted occasionally- or more than occasionally- in the various catholic blogs I read, asking for a prayer for the repose of his soul. I've usually just said whatever strikes me at the moment- saying the sign of the cross, a hail mary, or something off the cuff and not 'standardized' (I use the term for lack of a better one- like how the 'our father' and such have words already, i just say whatever's on my mind). I need to track down what prayer is recommended for the situation, or if there's one; there may be a specific one I'm just not aware of.