Integrate the Aggregation Manager in Business Intelligence Development Studio
Microsoft provides an Aggregation Manager sample application which gives an advanced interface for manually editing aggregations. We have taken this sample code and integrated it into BIDS Helper. This way, it can be run from Visual Studio when you're designing your cube, and aggregations you edit will be saved into your source code for the cube. (If you prefer, you can run the BIDS Helper version of Aggregation Manager live against a deployed cube if you open it through File... Open... Analysis Services Database.)
A number of bug fixes and enhancements were coded. Look at the complete list if you're interested.
To use Aggregation Manager, right click on the cube in Solution Explorer:

Then you are given windows which allow you to edit aggregations manually:

Appendix C of the performance guide provides documentation on how to use Aggregation Manager. The UI of Aggregation Manager inside BIDS is only slightly different in a few places.
Aggregation Manager can also be launched from the Partition tab of the cube designer:

(http://www.codeplex.com/bidshelper/Wiki/View.aspx?title=Aggregation%20Manager&referringTitle=Home )