Keeping records and knowing your win rate is of the best ways to improve winnings in texas holdem. It's not complex and takes no more than five minutes to do after playing for the day. The benefits are great compared to the effort.
First of all it allows you to see which games you are making the most money at. It's also a great confidence booster. Having confidence will improve your game even more. Especially on days where the cards seem unfair over and over. Just knowing how much you are making overall can help get through those ugly runs of cards, without playing scared or going on tilt.
Simply keep a daily record of how much you won or lost for the day, after you are done playing. Along with how many hours you played. And then every week or so add up the wins/loses to one total. And do the same for the hours. Then keep that info and continue keeping daily records. Eventually you will have enough hours played to give accurate statistics for that game.
The best way to calculate is by the hour.
Total win (or loss) divided by total hours = hourly win rate.
Make sure to note whether it is a + or - number. The more hours you play the more accurate it will be. For online games 200+ hours is a good start for cash games or single table tournaments. But for multi-table tournaments much more is needed since the swings are bigger. In live games about 600 hrs at minimum is best, since you cannot play multiple tables.
Also, the easiest way to keep records is play one type of game and stakes at a time. This will improve your game also. Focusing on one game at a time will help dominate holdem faster. And definitely makes keeping records easier. For example, play two $100 no limit cash games. Instead of a $100 NL cash game and a $100 tournament at the same time (online).
Also do not combine records for a $50 and $100 NL game. Keep a running log for both. Then you will know which game is actually better in the long run.
I have often found the similar results (for myself) in two different games. Example 20/40 limits = $42 p/h and 75/150= $44 p/h. There are just better players at the bigger games in general, not always though. So you may be surprised to find a smaller game actually offering an equal or better hourly rate than it's big brother, and less risk.