In my friend's place in Berlin, that I did up for him earlier this year (and which I will do a full post on soon) I bought some teeny-tiny bulldog clips and drew a very straight line on the wall, using a spirit level. I hammered in some little nails, pointing upwards slightly, to make a 'V' shape with the wall (so the bulldog clips don't slip off) and then clipped and hung a set of cards from them. Simple, but I quite liked the effect.
Amazon, looks good too (see a creative way to use that on Grace Bonney's ever excellent Design Sponge blog) and string hung vertically from a nail, or looped horizontally like bunting, with cards pegged along its length, is another of many ideas to try.
The cards were from the excellent bumper box set of Penguin postcards, £14.99 for 100, you can get as classics (like these ones) or as Ladybird or Puffin book covers (see below, same price) the children's imprints.
I'm also very partial to a classic old 60s or 70s John Hinde postcard, below, which you can pick up on eBay for a few quid a-piece.
But generally, I just can't resist a good new card. And this set of 1960s, 70s and 80s Royals postcards from Thorsten Van Elten really appealed.