A great way to re-use bottle caps and a good excuse to drink alot :)
I mainly used beer bottle tops for this but anything would look good.
If you don't drink or don't buy drinks with bottle tops, a good way to get them is to politely ask the barman at the pub for leftover bottle caps. Although these tend to be bent, they can still be used
Paint your mirror so it's all the one colour (if it's not already painted.) Having a busy background with more than one colour will make it look messy and take the eye away from the bottle tops.
Put all your bottle tops down first so you know they all fit, then get your glue gun ready and put a line of glue around the circumference of the bottle cap and stick it down onto the mirror.
Lay your mirror down on a flat surface to dry and then hang it up! Drying time varies but make sure its completely dry before hanging up, otherwise the caps might slide out of place.