I love using postage stamps in my collages. Each one is a tiny masterpiece with beautiful colors and elegant design. Use a special one, like the elephant stamp to suggest a theme for your design. The mushroom stamp suggested a great color scheme. Whistler's Mother with her austere demeanor was lightened up with the tongue-in-cheek pink pig. The elegant outfit on the angel stamp suggested the red and gold colors and theme for the ATC. On the blue and green ATC, postage created a beautiful background.
Many stamps are sold in batches by theme or country. Use the best ones for focal points in a piece. Since they are so small, layering to several different pieces of paper is an easy way to increase the size and importance of the stamp. The quirky ones, like the pink pig, can add a little humor and irreverance to your work. Finally, all of the basic nondescript ones make great backgrounds. Glue them down randomly or in rows. Most stamps here in the US are self stick now so they are like great little stickers- fun and easy to use.