I write down EVERYTHING I eat, every day. Without my food diary, it's too easy to forget the odd extra (or worse, convince myself that it doesn't count). Started doing it after I only lost half a lb one week back in May.
Back history is diet-controlled Type 2 for 8 years, allowed diet to get out of control, gained weight, started Metformin 500gx2 in March, joined Slimming World in April and started cycling to work, have lost 3st 9lb and 2-3 dress sizes in 6 months. Asthma much better, BP now a little on the low side (from high), BG normal, probably need to talk to doc about reducing drugs (electrolytes a bit out of kilter).
Still need to lose 4st 3lb to squeak into the top end of 'healthy' BMI range, but not sure it's acheiveable because I've never in my whole life been that!