TeamSugar member aistea311 recently posted one of my favorite tips. I live by this one every day and definitely think it's worth mentioning.
To spruce up an ordinary bottle of water, add fresh squeezed lime juice. For extra kick, do like aistea311 and add a few packets of Splenda and a splash of club soda.
While it is a super simple tip, it's definitely one worth keeping in your repertoire. If you don't have fresh lemons or limes on hand, you should consider keeping a bottle of 100% lime or lemon juice on hand. I personally have both, and find that it's a nice treat to be able to spruce up my water. Plus, guests are usually wowed when they ask for water and you say, "plain, with lemon, or with lime?"