I just do it on line, but you may need to hunt a bit to find the right confirmation number. Sometimes is is not the 6 alpha numeric number on the reservation - especially if it is a code share flight (say a flight booked with United that is operated by Air Canada). Some websites will automatically transfer you to the correct site (Delta did this with WestJet), but most don't. For a code share you may have to phone the operating airline. Personally, I'll book my premium or business seats as soon as I can, but for seats I have to pay for I will monitor how the flight is filling up and the opportunity to obtain my desired seats for free. Both hubby and I like aisles, so on 3x3 flights we'll try to sit across from one another or, if it's a long flight I'll sit behind him because he likes to recline his seat and I know it annoys people. In 2x2 configuration we will sit together unless it's a long flight, again I'll sit behind him so he can recline.