moon demon - It really depends on what kind of concert they're putting on, I guess.
First thing: If you go to buy in person, it would be best to be at a Ticketmaster outlet (preferably one that not too many people know about) as early as possible - like an hour before tickets go on sale. However, sometimes it doesn't matter if you're there super early because they'll give everyone numbers and then call them out in random order, so you could potentially get there first but end up buying last. I have never experienced this, though. You should also get your friends and family to buy tickets on line or over the phone at the same time, just in case you don't have the best of luck by yourself. If you end up with extra tickets, you can always sell them. This is the counter benefit from a high demand show like U2. Though tickets are tough to get, this also means you'll have no trouble getting rid of them. If you get your friends/relatives to help you buy tickets, you should put a cap on what type of ticket you're willing to buy. You don't want them buying you nose bleed seats if you don't want them to.
I usually buy through the ticketmaster website. You need a credit card for this. If you do this, you should register first, as you likely won't have enough time to enter all your information in on the day of sale. That's the simplest way to do it. Then you just go on the site, enter your password, and purchase your tickets as soon as they go on sale.
As for the concert itself, this is where it really depends on the kind of show they'll have. If they copy the Elevation Tour format and offer General Admission tickets for the floor, those are the ones to get (if you want to get close to the band). If you get those tickets - which were actually cheaper last tour than the actual seat tickets - you would want to get to the arena/stadium as early as possible, if you want to get close to the stage. I recommend 9 hours early. You can make a day of it if you're going with your friends. Bring some music (preferably a U2 mix CD), snacks, drinks, and some things you want signed by the band... because they will often come outside at some point before the concert. But I wouldn't spend the night at the arena or anything like that!