I started a feeding log with DD a few days after we got back from hospital. Not out of any concern for her weight, but simply to try and see a pattern to her feeding. For the first 6 weeks or so there was hardly any pattern (demand feeding mainly) but it became very useful when I decided I needed more routine/structure to her day.
She is now 15 weeks and I use it daily! I just write down the time of each feed, and how long she fed for. Then I know that she should not be due another feed until 2.5 hours after the start of the last one. I have a terrible memory so it's good to keep a log of it all.
Also, as a first-time Mum I find it really useful, because when DD starts to cry around 2 hours or so after she last fed, I can confidently say she's hungry, rather than play endless guesswork..
I also log her sleeps - just start and end times of each nap during the day. Then I know that if she cries around 2 hours after she last woke up, chances are she's tired (not hungry).
I am a VERY relaxed person and not normally the type to be this anal
but it really helped me structure my day, and now I know exactly when I have a 'window' to pop out and do something, which I know won't be interrupted by DD howling.
Once I started DD on a regular nap and feed routine she also changed OVERNIGHT into a much happier baby, and started sleeping through the night (7pm to 6am) at 9 weeks.
I just use a notepad and leave it out on the kitchen counter. If anyone else takes over baby duties they note feed and sleep times too. Easy peasy!
I think in the early days I spent a lot of time sticking my boob in DD's mouth every time she cried. With hindsight, I think she was actually simply over-tired. If I had been logging sleep and feeds, I wouldn't have made that mistake. Well worth doing in my view.