Cleaning your flag pole will require different methods based on whether the pole has experienced oxidization.
Nonoxidized
Aluminum flag poles are normally treated in order to resist oxidation, a process which leads to rust in the flagpole. As long as the process hasn't begun, cleaning the flag pole is simple. You should remove any debris that has gathered on the pole. Some scraping might be required, but care should be taken not to damage the protective coating when doing so. After removing debris from the pole, often a warmed soap and water solution can clean the pole completely. If the pole has tar or other difficult-to-remove substances, use a solvent that's specific to the substance requiring removal. Solvents should be tested in an unobtrusive area if the protective coating on the flag pole is a painted one, in order to avoid damage to the paint.
Oxidized Aluminum
When aluminum oxidizes, pitting or black discoloration will occur. In this event, you'll have to put a bit more effort and apply some harsher chemical methods to clean the surface. If your flag pole has become pitted, you'll have to sand the pitted areas smooth before continuing. Use 400-grit sandpaper for this, rinsing the surface and allowing it to dry completely when the area has been smoothed. Polish the surface with a commercial aluminum polish and cleaner. Mothers Mag and Aluminum Polish do a good job of cleaning aluminum surfaces that are no longer protected by a layer of lacquer or paint. Apply the polish with a damp applicator, then buff it off when it's nearly dry. Mothers Polish can be found at most auto parts and hardware stores. Another polish useful for an aluminum flag poles is Never-Dull, available from most hardware stores. Never-Dull comes on a specially treated cotton wadding cloth. You rub the cloth over your aluminum flag pole and then wipe the pole with a clean dry rag.
After Cleaning
While polish will remove corrosion and the effects of oxidation, it might also remove what protective coating remains on your flag pole, so the pole will have to be treated after it's polished. To protect the flag pole after it's been cleaned with harsher chemicals, you should apply a new protection layer. A coat of bees wax or a lacquer clear coat will provide protection from the elements and make it easier to clean when you have to clean it again.