I've used the summary index for maintaining summarized data long term. By summarizing it, you are reducing the size of the data, and therefore are able to store it longer without any modifications to retention.
If you are looking at legal retention, where the data has to be in original form, then I'd suggest defining your frozen areas for the indexes in lieu of the summary index. I believe once you dump stuff out to a frozen index, that file can be archived off (i.e. moved to tape), but please check that first. See below;
http://docs.splunk.com/Documentation/Splunk/6.0.3/Indexer/Setaretirementandarchivingpolicy
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