If you Diet Coke has caffeine then it actually means you need to drink more water to offset it. That's why I keep my hot tea with caffeine to one teabag and then switch to decaf. The Crystal Light is good but you really need plain water. There have been lots of posts that explain why it is so important. You can add lemon.
What has helped me lately has been to post in the Behavior goals and note exactly how much water and tea I have been drinking. I also make sure I finish 20 ounces of water before I go and refill my tea. Eating spicy food really helps me to consume more water. I drink a lot then.
I've started keeping 20 ounce bottles of water with me everywhere so I can sip on them. Find the temperature that suits you. In the summer, I like my water icy but this time of year, I prefer room temperature.
And take a measuring cup and use it to measure how much your cup will hold so you know how much it is. I used to think my glass at work was 32 ounces but that was wishful thinking.