Some cool ideas for where to place the things:
CARDS AND FLYERS
In people's hands.
In people's pockets when they're not looking. (reverse pick-pocketing!)
In the crack between traffic signs and their poles, so that the cards stand up straight.
On bus stop benches and window sills.
On top of newspaper boxes.
On top of library book drop bins.
Bus/train stations, doctor's offices, anywhere people are waiting and bored.
Tree branches along sidewalks.
Any intriguing crevice that appears at eye level.
Basically anywhere that people aren't expecting to see a book cover. Try to avoid the cluttered community boards and tables whenever possible. Go off the beaten path.
Be a bit more civil with these, as they are large and intrusive and I don't want to be a menace.
Community boards in coffee shops, libraries, book stores, etc. (ask permission!)
Power utility boxes.
Bridge underpasses, tunnels, places you see other posters.
Unexpected places, but not if it feels like you're littering in an otherwise pristine space.
Get creative! Post pics of your shenanigans!