The problem you have described is fairly common and could be related to one of two things. One is general hygiene. The other is being excessively sweaty, in addition to having a lot of anal hair. If you are the type who maintains good hygiene levels, this should not really be a problem. Cleanliness is an excellent way to get rid of all itching and sweating problems. You could start by paying special attention to cleaning this particular part of your body while having a bath. Use lots of soap and use your hands to scrub well and hard. Beyond a point, this type of bathing should become second nature to you. Remember that simply using toilet paper does not guarantee a good clean up. It is equally necessary to wash yourself extremely well. If you are the type of person who has a lot of anal hair, this will become exceptionally important. There are particles of matter that can get stuck in your anal hair and can cause you to really itch badly. You should try and see if you can keep a separate loofah or a washcloth of some sort to clean out this particular area.
If you are the type of person who perspires a lot also, you will find that you have this problem almost all the time. Sweat will have a tendency to trickle down your back and settle towards the anal cavity. This is the place that is the deepest crevice on your back and water will always run to the deepest possible level. If this is happening, you should also be aware that there will also be salt deposits that will contribute towards causing irritation. You should concentrate more on getting rid of the salt while washing as well. Apart from this, you can also make sure you do not sweat too much. One of the best ways to ensure this is to apply liberal amounts of talcum powder. This should particularly be applied all over your shoulders and back. These are the areas where you will find you sweat the most and from where the sweat will trickle down. You could also shower off your grime twice a day for effectiveness. This would become all the more important if you live an active lifestyle. If you are a sportsperson, for instance, or if you spend your time out in the sun where you would typically sweat a lot.