The more I think about it, the more I'd like to encourage beginners to start by working with wood. I use cardboard primarily because I work with a lot of signs, but if you're just going to stick up a sign or two to begin with, I think wood's probably the way to go. Find some scrap wood, paint it white and then paint on your message. Then go lean it up against something or nail it up somewhere. If you're nervous, do it at night. If you'd like your sign to stay up for awhile, use a ladder.
On fast sections of freeway, pay attention to bits of fencing and infrastructure that appear and disappear quickly. These are good places to post because by the time they see it, drivers are going too fast to do anything about it. This is just off the 5 northbound in Del Mar. It'll probably be there for awhile.
This sign, in Oceanside, is one I'm exceptionally proud of. Even though it's lying on the ground, the message is visible and legible to four or five lanes of traffic. Because it's lying on the ground however, it has the appearance of a sign that's already been taken down. Nevertheless, the message still comes across. I imagine this to be particularly maddening to the person who has to decide whether or not they're going to take the time to get it out of sight entirely.
I put this up on one of my favorite overpasses in LA two nights ago. It's tilted to get the full effect of an overhead light and sits squarely over six lanes of continually heavy traffic. It's a (poorly) sealed-off pedestrian overpass at 8th St. between Crenshaw and Arlington off the 10 that still has a working light and is perfect for any kind of posting day or night. Enter from the north end.