Getting your Ring Size Right!
The absolute best way to get an accurate measurement of your ring size is to be sized by a jeweller.
It should be free, can be done in a lunch break, and is the best way to make sure that you receive a ring that is the perfect fit for you. This is especially important if you're buying an expensive ring, or a wider ring - its normal to need a slightly bigger size due to the thickness of the band.
However if you can't get to a jeweller, you can size yourself using a ring that already fits well and a good chart. When using a chart its VITAL that you make sure you print it out at lifesize scale or 100% size. There's a decent one on the Michael Hill website which provides a ruler measurement so you can be sure that it is printed to the correct scale:
Measuring Someone Else's Ring Size in Secret...
Just so we're clear, this technique is not terribly accurate - it can give you a reasonable ball park guess though. Make sure you're measuring a ring that fits on the right finger :)
You'll need to be able to get to a jeweller's to use this method: put your partner's ring on your finger. Pick a finger where the ring fits as close to a normal position as possible. Make two marks, one on either side of the ring. Leave them there and go to a jeweller and tell them you want to find out what size ring goes on your finger where these marks are. Make sure it goes past a knuckle as this is eventually going to happen with the real ring.