We all love to do it, who wouldn't? Butts are fantastic. I don't even care if it's a dog, a person, or even a mouse (yes, a mouse, but just once when I was experimenting after racing). You can't just go right in like a maniac. You're not going to win any friends that way. You have to start politely, and then see where it goes from there.
The Technique
Just remember these few points and you'll be fine.
1) First of all, the most important thing to keep in mind, is make sure the owner of the butt you're interested knows you're there. I like to do a quick little circle around the whole dog (usually) to make sure everyone is aware that I'm here, and that I have intentions of getting to know you.
2) A quick look in the eye says "You can trust me." Don't be afraid to make eye contact. Heck, I even winked once.
3) Standing up, go sniff that butt; Sitting down, leave it on the ground. I like a good butt-sniff as much as the next dog, but it's just not worth getting dirty over. There are always other opportunities. Be patient. Dirt in your nose is WAY not fun... and then there always the chance that the butt is currently "in use." Just play it safe.
4) Work front to back. Start with the nose, make your way to the tail. You don't have dessert before dinner, do you?
5) Don't be selfish. It's fine to do your share of sniffing, but make yourself available too. In fact, go the extra mile. Since I'm so tall, I like to lean down a little bit sometimes for the smaller dogs to see what a real role model smells like.
Sticky Situations
We hate to admit it, but sometimes we accidently come upon some substance that we probably wish we hadn't gotten into. Fortunately, though, I have a tip for those of you who have this problem in the future. Since the people are much better at cleaning us than we are, make sure they notice the foreign substance. The best way I've found to get them to notice is to rub your head (or other infected area) on the carpets and furniture. You're sure to get cleaned ASAP!