ALWAYS KEEP A FOOD DIARY
Food diaries aren't just important on a daily basis to monitor progress, they could be essential to your long-term progress years from now. Every time you introduce a new variable to your diet (this also goes for training and supplementation) you should keep note, outlining how your body is responding on a daily basis.
Here is why...
There is no better way than to learn what is optimal for YOUR body than monitoring how you respond to new variables. For instance, if you do a 12-16 week diet having all the changes you made along the way saved will help with future decisions. You have proof of how quickly your body dropped weight, over what time and after what changes... This information is hugely valuable because it means you learn how to optimise things for the future.