Search engine optimisaton is not just a one-off activity; it's a continual battle. Methods change as search engines become more and more sophisticated. As Google themselves state on their web site, results can never be guaranteed (excluding pay per click, in which case it's costing you to be there). Any company telling you this should be avoided. Free or "natural listings" as they are known change position frequently so just because you are ranking well today does not guarantee you will rank well tomorrow.