I'd been thinking about it for AGES (about 3 years and trying unsuccessfully), but when the scale hit 14 stone 7 and my size 16s didn't fit anymore (so I was wearing about 3 sets of clothes on a cycle), and I worked out that my horse wasn't capable of carrying me properly any more (he groaned when I got on), and seeing a picture of me with my mates and not recognising myself.....
All these things came together in one week and I though "That's IT!". I rang LL the next day and started the following week. If only I'd done it sooner!