Here's a question I get asked quite a bit: when and how do you sign the marriage license? I've seen it done a number of ways.
(First though, make sure you go in person to the county office and pick up your marriage license within 3 and 60 days before the wedding! You can start the process online in Multnomah County, but you both still must go in person to pick it up, with valid ID. More about that here.)
You will have given the county clerk $60 and your personal information, and signed your names to a triplicate form. The license is then given to you to wait until the day of the wedding when your witnesses will print their names, your officiant will sign it and add his or her contact information, and make the license legally binding.
There's also a commemorative license. This is the "pretty" certificate that is included along with your marriage license, and you will want to also have your witnesses, officiant, and yourselves sign this too. It's just for looks though, and is NOT proof of legal marriage. (It is fun to get your witnesses and officiant to sign that one too, and to flash it on your honeymoon, in hopes of getting freebies :-)
So, again, when does all this happen? If you don't mind not having the photographer there, you can cheat a little and get it out of the way by signing everything the night before the wedding. Usually your officiant and witnesses are present at the rehearsal dinner, so this makes things really easy. This is a little unorthodox though, since you are technically now married the day before the wedding. However, it does make for one less thing to worry about on the day of.
The next option is to pre-fill out all the paperwork and witness names, and let the officiant sign on the day of. This way you are still officially executing the document on the actual day of your wedding, but it still cuts down on the paperwork and poring-over-fine-print part.
The final option is still the most traditional; gather your 2 adult witnesses and officiant on the day of the wedding, either right before you walk down the aisle in the dressing room, or right after, and do the form filling and formal signing with your photographer present. Just make sure to decide where in the schedule this will fall, and that the witnesses know, and pick a good location with a table and light where all this can take place. With the triplicate form and the commemorative license, it can amount to about 10-15 minutes of careful paperwork and can take up to 20-25 minutes out of the wedding day schedule.
And that's it! Just make sure that your officiant does MAIL the license back to the county within the following week. That final step ensures that your marriage is recorded by the government and you can have your happily ever after recognized in the eyes of the law as well as your family and friends.