If you don't want any colour cast, use a white card. If you want warm effects, use gold paper. If you want more directional lighting, ie not very diffused, use shiney surface. Other types of reflective material you might like to try is alluminium foil, crushed for a softer effect. Most importantly, experiment and see which gives you the effect you want or like.
Huggable's got a pack of 0.7mm plastic sheets from Artfriend, and a set of velcro strips from Spotlight. Cut the plastic sheet to size, and paste some velcro onto the lower edge. Paste the velcro round the tip of the flash, and volva, a bounce card!
Trying using a mouse pad for a making a bounce card. You know those thin stiff sheets with white shiny backing that became popular when optical mouses came out? Yeah they're perfect (Not the thick paded ones!)
Advantages in using white plastic bag as diffuser.
1)Easily available and free
2)Makes ur flash head waterproof
3)Can use as escape device when u botched a shoot midway. ( cover yr head n face with it and run )